Sennarth, Kurog Grimtotem's most vicious creation, is a massive arachnid infused with the unforgiving cold. Her singular purpose is to guard her brood of thousands, awaiting the day they'll hatch and enshroud Azeroth in a web of winter.
Sennarth is a multi-platform encounter.
You will first fight her on the floor of her chamber, and after she does one cycle of abilities, she will climb to the next platform, where she will remain for another cycle of abilities. This process repeats until she reaches the top of the room, where she will land and modify her abilities.
On each Platform, Sennarth will summon a Frostbreath Arachnid to help her.
Fight her to the top of the staircase to reach the final phase and defeat her.
Sennarth will automatically move to the top of the staircase when she reaches 5% Health.
Icy Ground [Be Careful]- Standing on Icy Ground will deal Frost damage every second whilst you are in it. You will also slide around making movement harder.
Chilling Blast [Spread]- When this ability is cast, spread out and avoid cleaving each other with the 
Frost Expulsion explosion at the end.
Enveloping Webs [Run Away]- You will drop webs on your location. Run away to place them for other players in your raid to use and avoid being hit by your own webs.
Sticky Webbing reduces your movement speed by 25% for 1 minute, however it prevents you from being displaced and sliding. This debuff can be removed by 
Caustic Eruption.
When you obtain Sticky Webbing 10 times you will be 
Wrapped in Webs which will slow your movement speed by 75%.
[Heroic/Mythic Only]: Wrapped in Webs will stun you for 30 seconds. This debuff can be removed by Caustic Eruption.
Gossamer Burst [Run away and get pulled into a web]- All players will be pulled towards the boss and take a large amount of Physical damage.
Web Blast [Tank Swap]- This is a stacking debuff from special attacks from Sennarth. Each time you are hit by it you will take 50% more damage from Web Blasts for 30 seconds. 5/6 stack swap. Taunt when your debuff runs out.
Apex of Ice [Interrupt]- When Sennarth reaches the top of the staircase she will channel this spell. Whilst this is being channelled Sennarth will take 99% less damage and deal increasing damage to the raid.
This also creates Icy Ground on the final platform.
In this Phase Sennarth keeps Chilling Blast and continues to spawn Caustic Spiderlings.
• Enveloping Webs is now 
Suffocating Webs.
• Gossamer Burst is now 
Repelling Burst.
• Chilling Blast now creates a 
Glacial Plume when it expires.
Suffocating Webs - At the end of the 6 second debuff the players targeted will now be knocked away. Sticky Webbing will spawn at their locations, and nearby players will be Wrapped in Webs.
Repelling Burst [Be close to the boss and don't face the edge]- This will push you away from the boss. Move close or get Sticky Webbing to avoid falling off the platform.
Glacial Plume [Avoid] - This creates knockback bombs on the ground at the location of every Chilling Blast circle.
Pervasive Cold - This buff is applied to the boss this phase which causes the raid to take 10% more Frost damage per tick until she dies.
Permafrost [Stay on the platform] - The ground below is now covered in Permafrost which deals stacking Frost damage every second whilst standing in it.
Web Blast cannot be slowed.
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Sennarth is a multi-platform encounter.
You will first fight her on the floor of her chamber, and after she does one cycle of abilities, she will climb to the next platform, where she will remain for another cycle of abilities. This process repeats until she reaches the top of the room, where she will land and modify her abilities.
On each Platform, Sennarth will summon a Frostbreath Arachnid to help her.
Fight her to the top of the staircase to reach the final phase and defeat her.
Sennarth will automatically move to the top of the staircase when she reaches 5% Health.
Bloodlust is best used on the pull. The more damage you do to Sennarth earlier in the fight, the shorter the final burn will be. You can even avoid it completely with enough DPS!
DPS players need to focus down the Frostbreath Arachnid as soon as it spawns. This should be tanked as close to the boss as possible to maximise on passive cleave damage.
Caustic Spiderlings spawn every 30 seconds. This lines up very well for Devastation Evokers so let them handle it and the rest of the raid DoTs them up and focus on the boss.
You know Sennarth will always climb to your left (facing the middle of the room). Start each phase either in the middle of the platform or to the left and slowly make your way across to avoid cluttering the room with webs and ice.
The main pieces of burst damage on this encounter are the push/pulls and Chilling Blast. You shouldn't need cooldowns on these until phase 3, a simple HoT ramp should cover the raid for the most part.
Make sure you stay in range of the boss and don't accidentally refresh your Web Blast stacks. Sennarth can target the other tank if you are out of range!
Breath of Ice - This will create patches of Icy Ground in the arena.
Icy Ground [Be Careful]- Standing on Icy Ground will deal Frost damage every second whilst you are in it. You will also slide around making movement harder.
Chilling Blast [Spread]- Several players will be targeted by this mechanic. They will take a small amount of Frost damage over 4 seconds.
When this effect expires or is removed Frost Expulsion will trigger. Frost Expulsion deals a high amount of Frost damage to all other players within 4 yards. Move away before it expires.
Enveloping Webs [Run Away]- Several players will be targeted by this mechanic. They will take a moderate amount of Physical damage over 6 seconds and then drop Sticky Webbing.
Sticky Webbing reduces your movement speed by 25% for 1 minute, however it prevents you from being displaced and sliding. This debuff can be removed by Caustic Eruption.
When you obtain Sticky Webbing 10 times you will be Wrapped in Webs which will slow your movement speed by 75%.
[Heroic/Mythic Only]: Wrapped in Webs will stun you for 30 seconds. This debuff can be removed by Caustic Eruption or any stun removal effect such as Blessing of Protection.
Gossamer Burst [Run away and get pulled into a web]- All players will be pulled towards the boss and take a large amount of Physical damage.
Web Blast [Tank Swap]- This is a stacking debuff from special attacks from Sennarth. Each time you are hit by it you will take 50% more damage from Web Blasts for 30 seconds. 5-8 stack swap. Taunt when your debuff runs out.
Apex of Ice [Interrupt]- When Sennarth reaches the top of the staircase she will channel this spell. Whilst this is being channelled Sennarth will take 99% less damage and deal increasing damage to the raid.
This also creates Icy Ground on the final platform.
In this Phase Sennarth keeps Chilling Blast and continue to spawn Caustic Spiderlings.
• Enveloping Webs is now Suffocating Webs.
• Gossamer Burst is now Repelling Burst.
• Chilling Blast now creates a Glacial Plume when it expires.
Suffocating Webs - At the end of the 6 second debuff the players targeted will now be knocked away. Sticky Webbing will spawn at their locations, and nearby players will be Wrapped in Webs.
Repelling Burst [Be close to the boss and don't face the edge]- This will push you away from the boss. Move close or get Sticky Webbing to avoid falling off the platform.
Glacial Plume [Avoid] - This creates knockback bombs on the ground at the location of every Chilling Blast circle.
Pervasive Cold - This buff is applied to the boss this phase which causes the raid to take 10% more Frost damage per tick until she dies.
Permafrost [Stay on the platform] - The ground below is now covered in Permafrost which deals stacking Frost damage every second whilst standing in it.
Web Blast cannot be slowed.
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This fight has three small adjustments on mythic to existing mechanics and one entirely new (and untested) mechanic:
Changes:
- Sticky Webbing movement speed reduction has been increased to 35%.
- Wrapped in Webs now happens on 8 stacks (down from 10).
- Caustic Eruption now applies a debuff which increases damage taken from it by 300% for 30 seconds.
New Mechanic:
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Gusting Rime is the new mechanic of the fight which creates pillars of ice that deal very large amounts of damage to players who come in contact with them and knock them away.
Detailed below are changes to your approach for the encounter on Mythic from Heroic.
Lust & Pot on pull and second pot when CDs are back in the last phase.
Frostbreath Arachnids need to die before the 5th tick of damage. These happen every 5 seconds from spawning meaning you have ~24 seconds to kill it. Ideally you would want to skip the 4th tick of damage as well. As a result of this a large amount of DPS focus is required for these.
Every DPS should be hitting these when they spawn with the exception of Arcane Mages who will lose too much damage from target swapping. However, if you raid needs them to swap then make them swap.
Your goal is to reach the final phase with Sennarth on ~32% or below.
You will need a Warlock Gateway on every staircase. The goal of these gateways are to let you climb the stairs faster and avoid most of the Gusting Rime spawns there.
Aim to not place webs where you are wanting to move to, this is especially important before the 2nd stair climb and the 3rd stair climb.
On the 2nd stair climb the webs will go out on players as you will intend to gateway. These players should stay behind and then gateway after their webs are done. If you have Paladins use Blessing of Freedom on these players.
On the 3rd stair climb the webs will go out after you gateway. These players should run to the sides of the stairs and place the webs there.
The main pieces of burst damage on this encounter are the push/pulls and Chilling Blast. You shouldn't need cooldowns on these until phase 3, a simple HoT ramp should cover the raid for the most part.
Your strategy for this boss will remain unchanged from Heroic but you are punished for poor web management with the reduction of Wrapped in Webs stacks required.
Your goal is still to get Sennarth as low as possible before the final burn. Optimising your DPS is key for this. Ensure the Frostbreath Arachnid is as close to the boss as possible for the free cleave DPS.
Gusting Rime is placed near ranged player locations. You should aim to start on the middle/right hand side of the room and when it spawns move along to the left.
You will need to place warlock gateways on the staircases to avoid most of the Gusting Rime spawns.
You will use lust and first pots here. As you have inflated damage here not everybody will need to be on the Frostbreath Arachnid. Assign a few DPS to hit the boss instead. Priority should be given to Arcane Mages and Unholy Death Knights.
Ideally your Warlocks should be playing Destruction on this encounter to gain additional boss damage.
As the add dies you will get your first AoE spread. This can be pre-immuned but should only really be done by Death Knights due to the high frequency of AMS.
You will then get your first pull into the boss. Either dip into a web at the last second or move towards the back of the room. Moving 2/3 the distance from the boss is enough.
It is important that tanks end up in melee range of the boss after this as if they are not here Sennarth will hit the highest threat player in range.
Slowly make your way towards the staircase and be ready for the second pull, the same rules apply here.
After this second pull your Warlock should be setting up the first gateway and the raid should be prepared for another AoE spread.
At the end of the gateway you should have some movement speed lined up either a Druid Roar or a Shaman Totem to get the raid to the top of the staircase ready for the next pull.
You can either use the rock near the boss side or run away from the boss, either work.
Your first few mechanics here are rather simple as its just an AoE spread, but then at ~1:35 the second Frostbreath Arachnid will spawn. This is the second hardest point of the fight. You will not have high cooldown availability from DPS so this add will live for longer than your first one. Have most of your DPS on this add and aim to kill it before or as the next AoE spread happens.
The rest of this platform plays the same as the start of the encounter where you will edge along towards the stairs, however there is one notable difference here. You will put Freedom effect(s) on the players who get the Webs before you gateway up the stairs.
When you reach the final platform you will have another pull in, however the boss positioning makes it a bit harder to use the rock to avoid the pull, so you should gather up on the ranged spot and use some kind of ground-based mitigation such as AMZ or Barrier.
The final add platform is here. This will play the same as the previous one however you will have 3 minute cooldowns back for the Frostbreath Arachnid. Save these and kill it quickly.
The gateway timing here is a bit different as you will need to gate sooner and then the players with webs move to the side of the staircase and you make your way to the top.
Once you reach the top of the stairs you will have an AoE spread followed by the final pull in.
There is nothing to keep you safe from this pull so you will need to run towards the ranged clump of players and get pulled towards the edge.
Your goal here is to have the boss as low as possible, anything below 35% is within kill range but I would recommend setting your DPS target to ~32%.
If you get bad RNG you will have a set of Chilling Blast which now create Glacial Plum soaks. Everyone soaks their own.
Interrupt the boss after the cast of Apex of Ice has happened, not during the wind up of the cast. This creates a cast sequence bug which is favorable to you and gives you more free DPS time.
You will then have Chilling Blast's go out on the raid. Once again these need to be soaked by the players that dropped them. After this you do not need to soak any more.
Everyone in the raid should have spread areas to put their Chilling Blast. Behind the boss should be used for melee, and the sides of the room for ranged.
Warlocks can place their personal portal on the platform. Jump off at ~1s remaining on Chilling Blast and teleport back on to save some space.
The two players with webs should stand under the boss and get pushed towards the stairs, this will leave webs under the boss but this is also space you will not use for the rest of the encounter. You can either kill a spiderling on them to free them or use Blessing of Protection (or other stun removal effects) to break the webs.
When the boss does a pushback use this to push yourself down the room and repeat the mechanics until the boss dies.
The longer the boss is in this phase the more damage you will take from Chilling Blast and the ticking damage on the ground. Use personals, healthstones, healing potions liberally from the second pushback onwards.
Web Blast cannot be slowed.
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