I can't seem to find any specifics on this question. When most monsters swallow a creature, the creature becomes restrained. If the Haunting Revenant swallows someone, do they become restrained? Do they have normal attacks? Do they get disadvantage on attacks? All I can see is to cast spells, you have to pass a con save and revenant gets advantage on attacks vs creatures inside it. Does this mean that melee fighters don't have any restrictions inside the revenant?

The stat block doesn't say that the creature becomes restrained, so it doesn't become restrained. However, since it does say that the creature can only get out by using interplanar travel, the restrained condition would be pretty redundant.

It also doesn't say they get disadvantage on their attacks, so they don't get disadvantage on their attacks. It does say that the swallowed creature has Total Cover against attacks and other effects outside the revenant, meaning that it can't attack or be attacked by anything outside. No reason it couldn't attack the revenant itself, though good luck doing that with a spell attack.

That's what I was saying. It looks like only magic users are effected inside of it. That just seems odd.