Legendary Creatures/Planeswalkers Matter Dominaria may support artifacts and sagas, but legendary creatures and planeswalkers are definitely the main focus of the set: not only do we have legendary creature support (Arvad the Cursed) and planeswalker support (Oath of Teferi) but we even got a brand new type of sorcery, the legendary sorcery (Primevals' Glorious Rebirth), which require you to have either legendary creatures or planeswalkers
I will admit it's a fairly derpy card in some ways, but the ability to steal the highest life total, or give the Archenemy the lowest life total is very strong in practice, and it can create really big swings for something like Exquisite Blood or Vito, Thorn of the Dusk Rose to generate even more value from
The more you have, the better I can usually do on price
From the original dual lands and the "Power Nine" to lesser-known gems and downright bad cards, the reserved list has it all
Shadow of Mortality gives this core a second threat to fill that role making Street Wraith and Dismember more appealing options