Highlord Kruul was a doomguard boss who appeared in various parts of Azeroth during the Dark Portal Opens event, introduced in patch 2.0.3. After the Dark Portal reopened, he officially replaced Lord Kazzak (now in Outland's Hellfire Peninsula) as the Burning Legion's field commander in Azeroth.
From the World of Warcraft news page under patch 2.0.3:
It is also reported that Lord Kazzak was seen departing through the Dark Portal. In his stead he has left his minion, Highlord Kruul, to sow fear and confusion among the defenders.
Accompanied by a host of infernals, Kruul has been seen near a number of major cities in Azeroth (a possible reference to the near legendary kites of his predecessor Doom Lord Kazzak), along with the following outdoor areas:
Note that this was ONLY a pre-BC event. Currently he has no known spawn points.
After finding Highlord Kruul you will need a well geared/skilled raid. The best places to engage him are the above zones, far away from major cities so that wandering players aren't killed (this avoids the 70k heal). Due to the short enrage timer, he will put out more damage, so he requires a larger raid with more healing, dps, and tanks to finish him quicker than Kazzak. Before pulling it is important to note a few things. First, there will be a flaming area near his spawn point that periodically releases level 50 Infernal Hound. They can be easily killed to avoid them attacking healers. Second, at each spawn there will be two boss infernals, these are easily pulled and killed away from Kruul and do not respawn. They also drop a BoE blue, so finding Kruul before he is killed could be quite profitable.
Most of his abilities are copies from Lord Kazzak, which is fitting since he is Kazzak's minion. He has yet to be observed using all of Kazzak's abilities.
- Melee – No heavy melee damage with his normal attacks.
- Shadowbolt Volley – Shoots out a long range cluster of shadow bolts for around 2k damage. Even when out of line of sight players will still be hit.
- Twisted Reflection – Magic debuff that heals Highlord Kruul for 25k hp every time he attacks the afflicted player. Also slows the affected player's movement speed by 50%.
- Thunderclap – An AOE nature spell hitting close range players. Low damage, and slows movement speed by 33%. Rumored to have another effect.
- Void Bolt – Single target shadowbolt Kruul casts on his target. This does around 3k damage.
- Capture Soul – When a player is killed, Highlord Kruul instantly gains 70k hp.
- Enrage – The biggest difference between Lord Kazzak and Highlord Kruul is the Enrage. Highlord Kruul enrages after 1 minute, where Kazzak was 3, making the fight quite a bit more difficult.
- Kazzak's Assault - Instantly kills Kruul's target. Only used against NPCs.
- Azeroth has cowered too long under our shadow! Now, feel the power of the Burning Crusade, and despair!
- Your fate is sealed, Azeroth! I will find the Aspect Shards, and then you will not stand against our might!
- Cower, little worms! Your heroes are nothing! Your saviors will be our first feast!
- Where? Where are the Shards! You cannot hide them from us!
- Your world will die, mortals! Your doom is now at hand!
Killing a player:
- Your own strength feeds me, <player>!
- Ha! This place is not yet worthy of my infliction.
Kazzak the Supreme
Legion Field Commander on Azeroth