This pack contains 5 pets:

Angry Lava - Boosts Doom damage.
Voilas - Boosts Void damage.
Muck - Boosts Gloom damage.
Darkoid - Boosts Shade damage.
Blood Spike - Boosts all damage types.

All 5 of them are passive pets and tier 1. You can upgrade all pets' tiers in game. You can't upgrade a tier of a pet if you don't own the pet.