Be sure to cure every detriment. This is a great fight to have your healers learn how to precast their cures.
In addition to keeping an eye on that specific aoe and making sure the one AOE from Putretor is also handled well you will also need to deal with an add that spawns on your raid.
The add is called "a ghostly dracurian warrior". This add has roughly 75,000,000 hp and must be burned down within 30 seconds or it will become "a timeless dracurian warrior" and will have over 500,000,000 hp. Best to just wipe the encounter at this point yourselves than deal with this addition.
The last bit of information on your way to completing this encounter is that Putretor has a Red Text Warning that must be jousted by any melee near him. The effect can be Stoneskinned or Death Prevented so your tank on Putretor does not have to move. If none are available then you will need a primary and secondary move spots for the group burning down Putretor. The text is, "The skeletal dragon begins thrashing with a toxic force that will prove fatal to anyone near!".
Joust the Red Text Warning, Cure Necrotic Rot right away and kill the add quickly. Grats on loot!
Trust me, they do not have to be burned down equally. Do it if you want to, but the extra damage is from not curing that aoe or no debuffs on a named.
Make sure you are fighting the two of them far enough apart from each other. If the two are too close together, bad things happen. We position along a line in the floor (a line perpendicular to the wall, not parallel), with MT tank spot against the wall.. his healers max range.. mage group max range from MT and the rest of the raid usually stacks there.. OT a ways farther down the line. This keeps Sevalor and Putretor far enough apart and everyone can reach who they need to. It also keeps the adds spawning in a centralized location so it's easy for the MA to target it and OT/3rd tank to grab it.
With our set up, on the joust only the MT group has to joust.
Our raid also has the MT group and the mage group dps on Putretor and the other groups on Sevalor until the adds spawn. At that time the mage group switches (with the rest of the raid except MT and MT group healers) to the adds. Once the adds are down, it's back to Sevalor and Putretor, rinse and repeat. We always end up taking Sevalor down first.
This is most likely due to necrotic rot not getting cured. If that cure gets missed, it buffs the damage that Putretor does... and buffs him a LOT. Our Inquisitor can solo heal me tanking Putretor while the rest of the raid force concentrates on Sevalor. If even one instance of necrotic rot expires on its own, then putretor will start ripping you apart. Its not a 100% fail condition though. The buff he gets is temporary. If you can last until it wears off, then his damage goes back to normal. Which tank is getting one shotted, the one on Sevalor or the one on Putretor?
Why does Putrid Breath keep randomly one shotting my tank for 120-140 k? Its not that its not getting cured, as i was personally responsible for that. Its the initial hit is destroying him.