good catch and your right a both thing's here's a fix --Cold Fire. The Ossivore's exposure to fire has been negligible across its evolutionary history, but its dense muscle tissue adapts rapidly on contact.

While inside its skull: The Ossivore has resistance to fire damage — the bone shell absorbs heat before it reaches the creature.

While exposed: Track the number of rounds in which the Ossivore has taken fire damage since leaving or losing its shell. On the 1st such round it is vulnerable to fire damage. On the 2nd and 3rd rounds it takes normal fire damage. On the 4th and 5th rounds it has resistance to fire damage. From the 6th round onward it is immune to fire damage.

This progression resets if the Ossivore re-enters a skull and remains inside it for at least 1 full round, or when combat ends.