The Tyrannosaurus Rex's Bite attack reads as follows:
Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit:
33 (4d12 + 7) piercing damage. If the target is a Medium or smaller
creature, it is grappled (escape DC 17). Until this grapple ends, the
target is restrained, and the tyrannosaurus can't bite another target.
Unlike 3.5e there doesn't seem to be any option to try to swallow the creature at this point. But it seems unlikely that the dinosaur would just hold/restrain the victim in its mouth. It says the tyrannosaurus can't bite another target, but can it keep biting the same target?
A logical guess would be that it does another 4d10+7 damage the next turn, without having to roll to hit, since it is already holding the creature between its teeth.
RAW though, I think that the dinosaur would still have to roll to hit, with advantage due to the victim being restrained.
What do you think the answer is RAW, and what would you rule as a DM?
Best Answer
The rules are pretty clear. The T-Rex can't bite another target, but there's certainly nothing stopping the T-Rex from biting the target it has in its mouth. Unlike, for example, the Stirge, the T-Rex doesn't have an auto-hit on a creature it is grappling, so it still has to roll to hit (with advantage, because of the restrained condition).
As far as what I would rule as a DM, I generally don't make rulings that contradict the rules unless I see a good reason for it. In this case, I'd probably say that a T-Rex has a pretty big mouth, and there's always a chance to roll out of the way of its teeth. I wouldn't convert its bite attack to an auto-hit, personally, unless I deliberately wanted to make it more dangerous. (And I think there's more interesting ways of doing that.)