Ok, I'm clueless about this boon. Up to level 6 spell on a barbarian that can be cast during rage.

So my DM has a mechanic in this game where we can get spells tattooed on us up to level 6. My level 10 barbarian just got a magic item that lets him cast and concentrate on spells during rage. But I am lost on what to choose.

Right now my best thought is Armor of Agathys at level 6.

Any ideas?