Feats (Dominaria Supplement)
From D&D Wiki
Toughness: Now adds 3 hp per level. Must have the back story or experiences to take it. Examples of backgrounds include being a military veteran (for characters who start past lvl 1), or being from the brutal Northlands when the rest of the party are from south of The Verge. As far as experiences something like surviving a few exceptionally horrific/rough fights (multiple PC deaths) or even a battle or two, or surviving crossing The Verge or the Arixan Wastes would be experience enough.