It's already in the official rules:
"The Servants of Glaaki can come into play – and their
special abilities take effect – even during the final battle,
although the Servants cannot directly participate in the
fight. At the end of the game, keep track of which
Servants of Glaaki were in play. Those Servants not in
play can still enter play as Glaaki’s attack raises the terror
level, causing Allies to be discarded. Once all of the
Servants have “entered play,” they have no further effect
on game play." (DH, p. 8)

There, uh, weren't any Clues on the board. (This can happen when you don't play with EVERY board. And this might have been one of my Kingsport games.) You know all those times you whipped Hastur's butt because you kept the Terror Track in check? Same thing, but without resistances, which really helps in getting double the successes. Beaten with Amanda, Lily (extra story boost), and William (monster trophies for Clue boost).
. 13 Doom track and if you use weapons, they are one-shot. With average rolls, you probably survive 2-3 attacks against him.