It is the crest of the Nightmare. You're able to summon in Nightmare as minions as you have complete the contract by feed the nightmare a magical creature(s).
You can summon a NIghtmare as minions as a bonus action and control them as a free action.
You can only have 2 Nightmares summoned at a time. You can summon 2 Nightmares per shot rest at level 1, 4 Nightmares per shot rest at level 7, 6 Nightmares per shot rest at the level at 12, and 8 at level 16.
You can resummon a Nightmares if it dies in battle as a bouse action but cost one of your summonses.