Hive Tyrant
A Hive Tyrant is the scariest possible thing an Imperial Guard will ever have the misfortune of seeing. Mostly because it will be the last thing he sees.
Hive Tyrants are a large type of the Tyranid Synapse-Creatures, making it three or four times the average human, all of it violent muscle, claws, and teeth able to tear through plasteel, ferrocrete, and ceramite with ease. Unlike other Tyranids, Hive Tyrants are unique entities in the Hive Hind, meaning that if someone actually does manage to kill one, it's not only coming back, but it's going to be seriously pissed off.
At least that's what the fluff says. On the tabletop, the Hive Tyrant has some interesting psychic powers, but is massively expensive, lacking invulnerable saves, and can't join squads. They're far from weak, in fact in close-combat they're flat out deadly, but they suck at shooting and lack much protection when getting shot at.
Dawn Of War
The Hive Tyrant appears as one of the tyranid heroes in Dawn of War II. Early game, due to its high starting health and DPS, it's the strongest commander in the game, and is immune to suppression, that it starts without a ranged weapon. Like the other tyranid commanders, the Hive Tyrant is extremely customizable, being able to preform a support synapse role, or become a dedicated melee fighter, though in the late game it's outperformed by the Warboss, Force Commander, and Chaos Lord.
Hive Tyrants appear as bosses in the vanilla Dawn of War II campaign and as one might expect, they can be tough opponents. The exception the Alpha Hive Tyrant that appears as a last minute final boss. While certainly not weak, by that point your squads are strong enough to the point it doesn't pose much threat, especially if you've got terminator armor from fighting the Warboss and Avatar (and if you beat either of those things, you can pwn the Alpha Hive Tyrant). The most ridiculous part of the fight that removes any chance of challenge from it is that Gabriel Angelos aids the player and can grant invulnerability to multiple units. Though, even with all the cheese you have, the Hive Tyrant Alpha is still a deadly opponent that you shouldn't be lax at in the fight. This is especially true in the Primarch difficulty as it's bio-plasma shot and charge can kill everyone in your squad and it only takes about 2-3 attacks for it to knock out one of your sergeants, even in full Terminator armor. So if you aren't careful in keeping your invulnerability skills up and deal enough damage to it while you're invulnerable, it can still wipe the floor with you.
That said, fans probably shouldn't have complained about how easy the Alpha Hive Tyrant was because the expansions included final bosses in form Ulkair and Kyras, who are both absolute hell.