This is from the Bungie Forum, thats how its done i guess:
Yes it's its own strike, 300 Darkblade with the Epic modifier. Everything is more or less the same up until the boss fight, where about every 20% into Alak-Hul's health a Yellow Knight will spawn. They're all unique, the first (For the Arc Sword) having an arc shield and sword, the second (For the Void Sword) having a void shield and boomer, and the third (For the Fire Sword) with an Fire Shield, Hive shredder, and the obnoxious ability to spam the flame font that Taken Knights throw out.
They're all fairly easy to kill (which could be considered a bad thing because of the whole 30 seconds to kill Alak-Hul)(Cursed thralls absolutely will kill them with splash damage), only taking about 10 swings with the sword tailored to their shields to down, so you essentially have to avoid all the ones you need for the quest chain until Alak-Hul is below 10%, kill the knight(first, we assumed if we killed Alak-Hul first they'd just despawn), then Alak-Hul.
The Arc blade R1/RT ability felt like it was described incorrectly, it fires more like a discus with range for almost any encounter in Destiny.
Edit: when anyone wants to do this with my just add: darkshodan84
but no mic at the moment