This information on Olm the Summoner, obtained from [url=http://www.mmo-champion.com/index.php?page=103]mmo-champion[/url], seeems to be a little more clear to me on the debuff removal by the enslaved felhounds.

[b:9f746f9ba6]Olm the Summoner (Warlock)[/b:9f746f9ba6]

[b:9f746f9ba6]Tanked by :[/b:9f746f9ba6] 2 Warriors or 1 Warrior + 1 Summoned Felhunter
[b:9f746f9ba6]Healers :[/b:9f746f9ba6] 3 (Shared with Priest)
[b:9f746f9ba6]Summon Felhunter[/b:9f746f9ba6]
Happens every minute. Enslave and taunt Olm with it. You can choose to let him tank if you don't have enough warriors, or just use the taunt and let him die. A warrior is more efficient to tank (he takes less damage) so if you can afford it you should let him die.  Do not kill him too fast if you don't need it to tank, he can dispell the Dark Decay DOT.
[b:9f746f9ba6]Death Coil[/b:9f746f9ba6]
Same as warlock spell, fear and deals 2000 damage to the main tank, also heal him for 4000 damage. This is where your 2nd tank taunt to get aggro.
[b:9f746f9ba6]Dark Decay[/b:9f746f9ba6]
DOT. 500 dmg every 2 seconds for 10 seconds. Not very threatening but if you've got a felhunter enslaved, use it to dispell it and save your healers some mana.