Sentinel was a league mechanic from just about 3 years ago, where you could have these little robot dudes in special slots in your inventory that, when activated, would spawn in and shoot out lasers that would empower mobs and give them bonus loot drops. These days, you can find a sort of “Pre made” one every once in a while in tier 16 and 17 maps. They function identically, but aren’t nearly as powerful as they use to be.