Guide to LotRO's Ost Dunhoth: Poison Wing
Having bested the oliphaunts, balrog and corrupted trees in the other wings of Ost Dunhoth, you have only one last wing to conquer before moving on to Ivar and eventually Gortheron himself: the Poison Wing. This wing becomes unlocked after battling your way through the corrupted garden and killing Baleleaf and Dourbark. It is a dank and soggy hell made corrupt and corrosive by powerful darkwaters. Likely, the trolls and pale-folk hanging around in the shallow acid pools aren't helping matters much either.
Poison is actually less of a concern here than acid damage. VIP players will probably want to spend some Destiny Points to buy acid mitigation, and gearing for acid defense is also a good idea.
FIRST BIG ROOM
The first group of mobs is in a large room with a number of platforms in the middle and relatively wide walkways around the edges. You will want to stay on these as much as possible, because the water surrounding them does moderate acid damage. It's not as strong here as it gets later on, but it's enough to make you want to consider staying out of it.
In the first room, you will be facing 4 Ninwaith Venom-keepers, 2 Raging Stone-trolls and 1 Dunhoth Lake-lord. The Lake-lord spawns in when the fight starts and is not visible at first, and will spawn Thralls during the fight. DPS will focus on the Venom-keepers, and Hunters will likely be forced to spam Purge Poison as these pale-folk run around attacking at random as pale-folk are wont to do.
The trolls have the dreaded Anger buff, so keeping them crowd-controlled (CC'ed) can be problematic. Much like the anger-buffed mobs in other wings, the strategy for dealing with these guys is to "double-tap" CC on them: hit them with one CC skill to dispel the Anger buff, wait for the buff to dispel, and hit them with a long mezz. This is the preferred method for dealing with the trolls, as they have attacks that apply a -50% incoming healing penalty to their targets which stacks with itself, and they have 360-degree area-effect attacks that can destroy groups fairly quickly. Keep them locked down and kill them one at a time.
Save the Lake-lord for last. When he falls, the water becomes less destructive - it still does some damage when standing in it, but less than it did.
There are probably a lot of ways to handle this second pull. This group consists of 4 trolls, several pale-folk and 2 Lake-lords, which spawn the thralls all through the fight. One method of more-easily dealing with this large of a group is to have the tanks run ahead and initiate the fight while everyone else stays back in the first big room, where there are lots of stable platforms to stand on. The tanks run the mobs back to the first room, and the constantly-spawning thralls stay in the second room, where they can be more easily dispatched later on without posing a constant threat to the healers. The other reason to fight in the first room is that the corrosive waters do significantly less damage there than they do in the second room.
Once the "lesser" mobs - the pale-folk and the trolls - are downed, head into the second room to destroy the Lake-lords. Healers should have room to stand on raised areas around the outside of the room, and an off-tank or two will want to stand close by on-call to take care of any thralls that come streaking for the belabored Minstrels and Rune-keepers keeping everyone alive.
After the second room is a "point of no return" drop, which leads into a large, flooded chamber where Gortheron is annoyed at being caught gloating in the shower.
He will trash-talk you for a bit, telling you about the keeper of the Darkthorn, a corruption that has slept in in a well far beneath the earth until just now, at which point Frothmar emerges from the foetid green waterfall.
There are a number of dirty tricks in this fight... unfortunately, most of them are the province of Frothmar and his minions. For starters, the water is very strongly acidic and has weakened the platforms around the room. This plays a major part in the fight - Frothmar himself presents somewhat less of a danger than the environment does.
The corrupted waters do progressively more damage the longer you stand in them, tiering up every 15 seconds. However, the acid also coats your weapons and causes you to deal more damage with attacks, and this damage bonus also tiers up the longer you stay in the water. There will come a point, though, where the damage you are taking from the water far outweighs the benefit from the extra damage, and you will need to step out and clear it. Clearing the acid is pretty easy - just dash up onto a nearby platform, wait about 4 seconds and it's gone.
This does, however, present something of a problem: the acid has weakened the stone of these platforms, and too much weight at once will cause the platforms to collapse, denying the group a precious safe-spot. On Tier 1 difficulty, having more than 2 characters on one platform will cause it to collapse. On Tier 2 difficulty, more than 1 person at a time is too much (even if they are both Hobbits).
There are 4 of these platforms in the room, so the raid can be divided into 4 groups of 3, and each group can be assigned to one of the platforms - Northeast, Northwest, Southeast, Southwest. Generally, the fighting will take place closer to the south platforms, so melee classes and main healers can be assigned to those two. Ranged attackers can be sent to the north platforms (the ones closer to the entrance to the room).
The group will want to keep cycling players on and off the platforms at all times. In the illustration above, each group rotates clockwise, so each person spends 30 seconds in the water and 15 seconds out. The red dots represent the tanks, who will need to be staggered in their platform rotation so that at least one of them is able to hold Frothmar at all times. Having two tanks is more or less mandatory for this fight - one tank cannot stay on Frothmar full-time in the water and expect to live, and will need to take a few seconds to dump aggro, clear the acid buildup and top off on heals. The tanks can be divided into separate groups (SW and SE), so long as the rotation keeps one of them facing Frothmar at all times. In the illustration above, the SE group's tank would start in the position immediately to the east of the left group's tank, and would be next in line for the SE platform.
For Tier 1 difficulty, this is less of an issue. Healers can stay parked on their platforms full-time, and the other 2 members of that group will take turns in and out of the acid. It only takes 4 seconds to clear the Corrosive Acid debuff, so a player can spend 40 - 45 seconds in the soup, step back to safety and clear it and then get right back in.
If you're not the guy tanking Frothmar, this is more likely to be the source of your doom.
Platform-cycling can be called out by one player who closely monitors the acid tiers. At tier 4, the acid does around 1000 points per tick, and that is a good time to switch. Alternately, the switch-caller can call out a switch every 15 seconds, keeping a regular pattern, and the group rotates one person on to the platform and everyone else off. For the Tier 2 challenge, each platform must have 1 person on it at all times throughout the fight - there is a very brief grace period where 2 people can be on at once, but take too long and the floor crumbles out from under you.
Another hazard in this fight is the adds that Frothmar calls in periodically to destroy the platforms. These are darkwaters with 9 - 10,00 morale, and they will immediately begin attacking the platforms they are sent to. They are single-minded and do not attack characters until and unless the platform is destroyed. These are priority targets when they pop up and must be killed very quickly before they smash a platform - especially on Tier 2 difficulty, where all the platforms must remain whole.
Frothmar is also fond of the field-goal and will punt characters around the room. When he does this, everyone will want to follow the arc of the punted character and be prepared to move if it looks like he will land on a platform. This is a ranged attack, and he may punt characters off of platform, leaving them unoccupied if nobody is prepared to move in quickly to take that spot.
This fight relies on precision timing and clear communication. It is not a DPS race (except when the darkwaters come in to smash the platforms), so the group can take their time against Frothmar. It also relies on the tanks' ability to quickly swap aggro - if the tank swapping out for the platform still has aggro and Frothmar follows him onto it, the platform will collapse.
When Frothmar falls, the last of the easy wings is defeated and the path before you leads next to Ivar the Bloodhand and his master, the ruler of Ost Dunhoth, Gortheron. Divy up the loot, take a long, steamy chemical shower and get plenty of rest. Play time is over, and hell awaits.