Grave Pact

In a Two-Headed Giant game, when a creature you control dies, Grave Pact's triggered ability will cause each other player, including your teammate, to sacrifice a creature. Grave Pact's ability won't trigger when a creature your teammate controls dies.

None of the players are targeted. A player with hexproof would still sacrifice a creature.

If two players each control a Grave Pact and a creature controlled by one of those players dies, a chain reaction happens. First the ability of that player's Grave Pact will trigger, causing each other player to sacrifice a creature. That sacrifice causes the ability of the other player's Grave Pact to trigger, and so on.