Thanatos Tower
Floor 4 - Sealed Room
Overview
The frenzy mechanic of this fight leads to easy wipes here. Pay attention to AoE's, specifically ones that appear right under your feat and try your best to not get infected while you burn this guy down.
You may want to watch a video guide to see things in action here. This helped me see things I didn't even realize I had to worry about when I was writing this up so this was just as informative for me as it will be for you.
Recommended Video Guide: https://www.youtube.com/watch?v=_jnzTx51Gvo&ab_channel=CherrySmith
Infection
Before we dive into the phases let's talk about the main mechanic of this fight, infection.
A bar is shown underneath your character that fills as you get more and more frenzy. When it reaches 100 (completely full) it will instant kill you and cause the boss to cast a massive AoE which also adds lots of frenzy to everyone in the raid and can easily wipe the raid, especially during Phase 2.
Phase 1 - Thanatos HP Above 60%
As I mentioned above keep your infection level as low as possible by avoiding AoE, don't stand in Nightmare Traps, and refresh buffs every minute as needed. The boss will use the following skills:
Purification Field
Every 60 seconds the boss casts this large AoE centered on him that removes all buffs and ground skills such as safety wall, fire pillars, etc
Remember to recast your buffs and fields as needed.
Nightmare Claw
Immobilize a random player for 20 seconds
A green circle will appear. Anyone stepping on this circle free's that player from the bind.
Nightmare Infection
Randomly marks 2 players with an AoE field that follows the player and explodes after 5 seconds and raises frenzy by 20.
If you have this run away from the party.
If players are standing still they may be immobilized by Nightmare Claw. Run away if the player is standing still since it may not be their fault.
Nightmare Trap
Boss drops an AoE field underneath 3 players that explode after 5 seconds dealing damage and adding 5 frenzy.
Nightmare Seed
Randomly debuffs a player for 5 seconds which deals damage and raises frenzy by 2 per second.
Other Spawns Include:
Sealed Orb
A yellow orb is dropped at a random spot on the map throughout the fight (visible from your minimap) that when destroyed, removes 5 frenzy per second to nearby players for 10 seconds. These continue to spawn throughout the entire fight and become vital during phase 2 to keep frenzy down.
Nightmare Executioners
During phase 1, if your frenzy reaches 40 a nightmare executioner will spawn. Just kill these off as you see them.
Phase 2a - Thanatos HP Less than 60%
Everyone's frenzy counter is reset at the start of this phase.
The entire map will be covered in infection except for the yellow aura in the center which is protects you from this. If you're standing outside the circle you gain 2 frenzy per second. Use those yellow sealed orbs to keep frenzy down among the party. Also be aware that the yellow aura may start shrinking if people are having trouble managing the mechanic in Phase 2b so be aware of this and adjust accordingly.
If your frenzy reaches 40 or more you will be teleported to another zone (see Phase 2b) away from the main boss which has its own mechanics to contend with. At the start of the fight someone is going to have to raise their frenzy enough to be teleported down there to mitigate the amount of Nightmare Outbursts that the boss will use. This is explained below in the Phase 2b section.
Care should be taken During this phase as Thanatos will continue using various moves from Phase 1 including Nightmare Trap and a few new ones such as:
Magic Sword Rush
A cross shaped AoE zone will appear centered on the boss that will activate after 3 seconds. This will cause damage, knockback, and add 10 frenzy.
Nightmare Outburst
Boss unleashes a massive AoE that explodes after 3 seconds that deals heavy dark damage and adds 20 frenzy. This can easily kill you but more importantly, adds half the frenzy needed to teleport you to the other zone so you want to avoid this at all costs if it goes off.
The boss only casts this when either:
A party member's frenzy reaches 100, which also instantly kills that person
A Nightmare Overlord reaches the center NPC in the other zone. See Phase 2b for details on this.
Frenzy Auto Attacks
Taking damage from the boss raises frenzy and thus, makes your tank susceptible to going over 40 frenzy leading them to the other zone to allow the boss to destroy the team. Priests should spam safety wall to prevent frenzy increases when tanking damage from the boss.
If you're a DPS and see a free safety wall field that's not occupied feel free to stand in it just in case the boss runs over to hit you to prevent frenzy build up.
For safety, consider bringing two tanks just in case one of them gets teleported.
The tank should keep an eye on sealed orbs spawns and move the boss as close as possible to keep the groups frenzy low so people that should be focused on killing the main boss don't get teleported to the other zone.
Phase 2b - Nightmare Core (Other Zone)
If your frenzy reached 40 you're teleported here. There is an NPC in the center of the room and two types of monsters will spawn: nightmare cralwers and nightmare overlord (Phreeoni). At the start of Phase 2 you'll need to send someone down here to deal with the Nightmare Overlords to prevent the boss from using the ability Nightmare Outburst. Typically this job is reserved for warlocks because:
HP Absorb can one hit kill the crawlers
Safety Wall and White Barrier mitigate damage preventing frenzy growth and allow you to survive
Nightmare Crawlers will work their way towards the NPC in the center and if they reach him everybody in this room will receive damage and adds 2 frenzy. The yellow aura on the main map will also get smaller as more crawlers explode so if you're strong enough try to kill these before they reach the NPC to make the teams job easier.
Nightmare Overlords are your prime target here since they will cause Thanatos will cast Nightmare Outburst on the party if they reach the NPC in the center. If you're teleported down here your primary job is to distract and/or kill the Nightmare Overlord. To make matters worse, taking damage from him raises your frenzy and you will inevitably reach 100 and wipe the raid so if you're not strong enough to burst it down, kite as best as you can to buy time or find a safety wall to stand in if someone places a bunch.
Siren offers a teleport back to the main zone on a 20 second cooldown that will completely reset your frenzy. If people's frenzy gets too high and they wind up down here them may use the teleport to escape. However, if you're down here to kill the boss and your frenzy is too high and they take the teleport then you're probably going to die. Be sure to communicate this so someone can take over and deal with the Nightmare Overlords before the team wipes.
Phase 3 - Run Away!
If you've defeated Thanatos and the Priestess spawned then you've pretty much won. Just run away from her for 30 seconds and you'll win as long as one person is still alive, easy money!