By which I mean start with one. It sets your Con to 19. You need one. Then you will be a god amongst men. Since you're playing Tomb of AnihilationAnnihilation you may want to know:
The Undying instead gives you a sanctuary-spell-like effect that's always on, but only functions against undead and only until they first make the save. Tomb of AnhinnialationAnnihilation has a lot of undead baddies, so this is pretty comparable, but that's all it's going to give you. Your call on whether saving the allowed-to-cast-spells-this-round flag for something else is worth it.
Half-orcs (and Minotaurs from Planescape:AmonkhetPlanescape: Amonkhet, but those swap out darkvision for a garbage natural weapon) get +1 constitution and the ability to instead be fine the first time they would be reduced to 0 hp or below each long rest. This doesn't trigger if you are instakilledinsta-killed by massive damage, which is a significant concern for you since that's, like, 45 damage and there are CR 6 creatures that can actually do that in a single round if you let them (don't get charge-stomped by a mammoth).
In combat, you want to be in the back and avoid damage as much as possible, using an unseen servant and/or flock of familiars and/or your sentinel raven to control enemy movement options. You can cast enlarge or false life on an ally to buff them without losing sanctuary, and you can cast sanctuary twice per short rest to discourage enemies from attacking you. Armor of Agathys can singlehandedlysingle-handedly eliminate hordes of weaker enemies who attack you, because your Arcane Ward will ensure that the temp hp from that spell stay around until you're basically dead anyways. Eldritch Blast provides an option to deal okayish ranged damage if it comes up, and something to do while sanctuarying when defence isn't urgent enough to warrant a blade ward.
