The turns of thawing blade seem to scale with damage done, though capped at 5. Might be damage done/4? (doesn't match al numbers here, so might be something like [2+dam/8] instead? to look into. (Or I did the math wrong, or got screwed up by the end of combat turn loss or something.)
You stab the serpent with your melting blade for 14 damage, but the serpent is already underwater. The water absorbs all but 6 damage. +3??
You stab the writhing mass with your melting blade for 20 damage. +5
You stab the writhing mass with your melting blade for 17 damage. +5
You stab the serpent with your melting blade for 13 damage. +4
You stab the serpent with your melting blade for 16 damage. +4 (-1 end of combat)
You stab the writhing mass with your melting blade for 14 damage. +4
You stab the writhing mass with your melting blade for 16 damage. +4
You stab the sludge with your melting blade for 21 damage. +5
You stab the writhing mass with your melting blade for 32 damage. +5 (-1 end of combat)