Where is a verb in "a troll out cold with a bloody lump on its head"

The whole portion you have emphasized is an adjective phrase, an adjective clause with the subject pronoun and verb omitted; here's the full sentence as context for other readers:

Eyes watering, they saw, flat on the floor in front of them, a troll even larger than the one they had tackled, out cold with a bloody lump on its head.

You can parse this as "...they saw, ... a troll" and the extra phrases all modify the object "a troll". In the specific case you're interested in, you can imagine adding "which was", so "... they saw, ... a troll ... which was out cold with a bloody lump on its head"

"out cold" is a phrase meaning, as the comment suggests, unconscious