A "creature source" is any object with the card type "creature." This means you could spend the mana to activate an ability of a creature card in your hand or graveyard
Mountain Rarity: Common TYPE: Basic Land SUBTYPE: Mountain CMC: 0 Mana cost: USD: $0.28 EUR: 0.12 TIX: 0.03 EDHREC RANK: - ( : Add .) Mountain Rarity: Common TYPE: Basic Land SUBTYPE: Mountain CMC: 0 Mana cost: USD: $0.28 EUR: 0.11 TIX: 0.03 EDHREC RANK: - ( : Add .) Mountain Rarity: Common TYPE: Basic Land SUBTYPE: Mountain CMC: 0 Mana cost: USD: $0.26 EUR: 0.13 TIX: 0.03 EDHREC RANK: - ( : Add .) Mountain Rarity: Common TYPE: Basic Land SUBTYPE: Mountain CMC: 0 Mana cost: USD: $0.33 EUR: 0.09 TIX: 0.03 EDHREC RANK: - ( : Add .) Mourner's Shield Rarity: Uncommon TYPE: Artifact SUBTYPE: - CMC: 4 Mana cost: USD: $0.15 EUR: 0.09 TIX: 0.03 EDHREC RANK: 26620 Imprint When this artifact enters, you may exile target card from a graveyard

Many of these cards are legendary creatures that depict Marvel characters, but theres also a land card for Xaviers School for the Gifted, an artifact for Magnetos helm, a token for Sentinels, and various sorcery spells that represent story elements that comic book readers would know
This is the easiest mechanic to understand of the five new mechanics and is consistent in all its applications by that it always triggers when the creature dies
Sorcery Color tag templates(None) Shamanic Revelation Draw a card for each creature you control
Its obviously not the strongest play, but its a nice way to play a snow deck