JoeCrow
Explorer
The way I'm probably gonna end up handling it is like this. (Game ended up getting cancelled this week, so I've got time to scheme.) During the full moon, he's full-on crazy murder machine, but he can make Wisdom saves (every round, increasing DC) to gain temporary control of himself. Other times, he can try and change at will, but stress/damage/other things can prompt rage, and then it's back to making Wisdom saves. Armor/equipment/clothes/etc: do not change. Damage immunities only work in changed form; might give him some sensory enhancements. Maybe advantage on scent-based rolls, that kind of thing. He's already got a higher base Strength than the change would give him. I dunno about the animals sensing him thing, I might give him that as a screwjob effect. I might give him advantage on Intimidate checks to balance it out. And yes, it's true immunity; the attacks don't bounce off, they heal immediately (and prompt Wisdom checks vs. flipping out and killing everybody).
As for why the immunity works on weapons and not natural stuff like rocks falling: it's a magical curse, folks. Why does Protection Against Missiles keep arrows from hitting you but not raindrops?
As for why the immunity works on weapons and not natural stuff like rocks falling: it's a magical curse, folks. Why does Protection Against Missiles keep arrows from hitting you but not raindrops?