Like most Magic: The Gathering cards, the value of Mortarion, Daemon Primarch depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Warhammer 40,000 Commander)
The creature deals the damage, so if it has lifelink, you gain life equal to its power times the number of creatures on the battlefield
b One black Japanese variant (no letter attached), four Neon Ink variants (a-d), one Serialized golden variant (f)
Whenever the Sylex is exiled you get to pay to tutor out a planeswalker and reveal it before putting it into your hand
That's really the main use case
I highly recommend this deck for anyone eager to learn the latest and greatest mechanics to come out of Lorcana, as well as to anyone looking to upgrade Amethyst/Steel into a stronger deck.