A Great Unclean One bound in planet Aurelia by Chapter Master Moriah of the Blood Ravens, Ulkair serves as the main antagonist of Chaos Rising, trying to get free and sacrifice the chapter to Chaos. Before turning to Khorne and becoming a pretty cool guy, Azariah Kyras first helped to bind Ulkair, then made a pact with him for power, after being shat out of the warp and onto the Space Hulk Judgement of Carrion, where Ulkair's power was greatest. Since the seal placed upon him was failing, he began corrupting subsector Aurelia, allowing traitors such as apothecary Galan to infiltrate the Blood Ravens.
Ulkair is mentioned throughout the second half of the campaign, when your squads see the amount of corruption within the Blood Ravens. One of his minions corrupted the apothecary Galan, and Ulkair himself made bargains with Azariah Kyras. He only appears properly in the last mission, where your men, Gabriel Angelos, and an assortment of
human shields unnamed marines launch an assault on Selenon Keep after softening it up with tanks.
The level of bullshit here is already high: you need to escort your brother marines through a gauntlet of Black Legion marines, Chaos Dreadnoughts, Predator tanks, and daemon-spewing shrines. If all of them die, Ulkair's health doubles; on Primarch difficulty, this might as well be a fact of life. Notably, the count isn't stopped when you fight him, and since Ulkair starts off with ridiculous health values as it is, you'd better get used to having the count suddenly skyrocket unless you destroyed every single temple and shrine beforehand, which is itself time-consuming and often ends with the redshirts dying anyway. The count also doesn't stop when you fight Eliphas, who handily takes the spot of Chaos Rising's second worst boss by spewing insta-kill doombolts everywhere and summoning swarms of Bloodletters onto you. Note that all of this occurs before the fight itself. This means that if you took the all-pure path and had Martellus tard out, you fight the 3 hardest bosses in the entire game in the span of 2 missions.
Once you get up close and personal with the fat cunt, it starts getting worse. Ulkair can vomit onto you, scattering your squads and doing the same amount of damage as artillery. If you try to kite him, he grabs you with his intestines, instantly killing whatever unlucky sergeant got caught and moving them to his body, making it all the more difficult to get them back. He also has a supercharged version of the sweeping doom attack that Eliphas gets in Retribution, which is liable to instantly kill anything caught in it except a terminator force commander or Tarkus, and has a massive blast radius. As if damage output to put the Avatar of Khaine to shame isn't enough, his health also hits the seven-digit mark. Like Martellus, he likes calling in heavy units as backup, to distract you and make everything worse. It adds up to a festering shitheap that makes the Hive Tyrant Alpha look like a defanged puppy. You can't just have Cyrus drop bombs and blow him up, either; he sees through stealth and has an area that causes them to instantly detonate and likely damage Cyrus while he's attempting to place them. He's arguably even worse than Kyras, since you could at least spam that fucker to death with troops.
Do note that despite how we've portrayed him, Ulkair IS beatable in harder difficulties, even in Primarch. It's only that it's very long and tedious to do so, requiring plenty of repetitive strategies and micro-management for around 20-30 minutes that, over time, it really starts to get boring. If you do want to take him down fast, however, bring plenty of dakka (as in everyone in your squad is capable of shooting): the battle is heavily inclined to shooty than choppy players. Of course, given the amount of stupidly game-breaking variety you get, it's not that there's any doubt of victory, it's that it takes so long.
On the whole, Ulkair is the crowning example of why Chaos Rising's bosses are stupid hard, taking the worst bits from everything you meet in the campaign.
Ulkair returns in Retribution as a corrupting influence still infesting Aurelia. He corrupts an Imperial Guard regiment stationed there and eventually had them turn traitor, which you have to eliminate as part of an optional mission. Why the Inquisition or the Guard thought it was a good idea to station guardsmen in a planet infested by a greater daemonic entity is unknown.
In the Chaos campaign though, Eliphas frees him, acting as the relic unit for the Chaos Space Marines. Much like Eliphas and Carron's daemon prince forms, he's no longer incredibly tough or strong, although he certainly does his job well and is especially devastating against vehicles (other than Kain with a lascannon, he's easily your best asset against metal bawkses). He doesn't have much bearing on the plot, however. He also talks in some of the missions and in the planetary briefings, which is a welcome addition, as only Eliphas and Neroth talk in that campaign. He seems to be far, far less powerful than the Khornate daemon of Tartarus (or at the very least an ascended Kyras) given the ridiculous disparity in power between him and Kyras. Though this does make narrative sense, the Daemon of Tartarus is essentially the arc villain of a plotline started in the very first game, while Ulkair was simply the main villain of one game; so having the closest thing Dawn of War has to a big bad be the most powerful villain encountered; even including Ulkair, is simply sensible writing. Not only that, but given he was destroyed and reawakened by Eliphas a few days ago, so he might still need to shake off the Daemon-sleepiness and get back in the swing of things with some tasty souls to juice him back up.
In canon, after the Great Rift, Ulkair was still kept in Aurelia, but is starting to stir quite a bit now.