We played a second session against the Lord of Spite tonight.
Long story short - it went the whole day, and the PCs managed to finish off the devil maws (now aka mawbörg) and doomchildren . . . but at some cost in lives. The Lord of Spite was driven back, but at the cost of several limbs. But the fight didn't end - the PCs tried to pursue, but carefully, and the Lord of Spite returned. Full details tomorrow, when I have time to write up Part II of III of Against the Lord of Spite.