Zek'vir is the secret final boss of the delves. He has his own delve called Zek'vir's Lair in the deepest part of the City of Threads and it consists of a single boss fight against him. The fight has two difficulties marked as ? and ??.
His fight involves a series of mechanics that can easily oneshot you if you are not properly geared for it and require you to use big part of your defensive and utility kit to deal with it. You aren't required to do a heavy amount of healing here but do need to handle each mechanic almost perfectly while keeping yourself healthy or you will die very quickly
Zek'vir's abilities
First there is 450451. This is a very obvious frontal cone and all you need to do is move out of it. Sometimes he will cast it twice back to back so keep an eye out for that. 450519 works similarly, it includes a grip but the end result if you get hit by it is the same: you die.
Hard mode adds a portal mechanic to 450519: The boss will summon three portals in the room while he's casting the 450519 and will shoot the web into one of the portals. The 450519 will come out of the other portal and connect with the final one, killing you if you get hit by any part of it in either side of the portals
450492 creates an aoe circle around Zek'vir that will fear anyone inside once the cast finishes. It is very easy for you to dodge but note that Brann will get feared part of the time, you can opt to dispell him to get him back to doing damage but be very careful about using your nat on Brann because it's vital that you use it properly for other mechanics.
On hard mode, 450492 will also send orbs in four direction around Zek'vir that are easy to dodge but you need to pay attention to.
450505 will apply a nature damage debuff on you and Brann, doing a heavy amount of damage. Brann won't have troubles surviving on his own but you need to dispell this from yourself as soon as possible to avoid the combination of this damage and the melee attacks from killing you.
You can also interrupt this cast and Brann will get several interrupts by himself, however in Hard Mode it's actually not possible for you to interrupt this ability and still kill the boss as you will need your interrupt to cancel 450914.
Zek'vir will cast 450568. This spawns a 224077 that can be attacked and does a 21 seconds channel. At the end of the channel if the 224077 is not killed it will spawn a Web Terror that has the abiliy 450597. If this cast goes off it will surely deal to a wipe with no way of recovering
This is the main dps check of the fight, as not killing the 224077s in time will very quickly lead to wipes. You should save resources and cooldowns to burst down these as efficiently as possible.
A final addition of Hard Mode is 450872, which rains down a series of swirlys in the ground that are deadly but decently easy to dodge, you can use hover to still maximize your dps during the mechanic.
TL;DR:
- Move out of 450451, 450519 and 450492
- Don't interrupt 450505, let it cast and dispell yourself as soon as possible after
- Interrupt every single 450914
- Kill the 224077, all your dps efforts should be focused on this
- Keep yourself and Brann healthy, you will be tanking the boss and if you die the encounter resets