Originally posted by LastChime:

That event is started by killing Eikthyr (first boss) and ended by slaying the Elder (second boss), as others have indicated you have to stay in that ring to burn the time on it. Tends to take them a while to even chew through stakewall, so maybe build a small enclosure with elevated firing platforms.

Exactly this. Each Boss you slay triggers the attacks of the minions of the next one. So make sure you defences can withstand the attack before you take on The Elder as when he goes down you ramp up the opposition! Word to the wise!