Nowadays, ETBs are printed in massive quantities, but this wasn't the case in the Sun & Moon era, when ETBs weren't in high demand
The card does something fairly unique in white: it steals creatures
Inspired by the legendary Black Lotus, it can generate three mana of a single color, but it can only be used to cast the commander
In Mews particular case, there isn't even the question to be asked whether it will survive the rotation, because the entire deck rotates
Liliana Vess Liliana of the Dark Realms Liliana of the Veil Liliana's Caress Liliana's Contract Liliana's Defeat Liliana's Devotee Liliana's Elite Liliana's Indignation Liliana's Influence Liliana's Mastery Liliana's Other Contract (double-faced)(playtest) Liliana's Reaver Liliana's Scorn Liliana's Scrounger Liliana's Shade Liliana's Specter Liliana's Spoils Liliana's Standard Bearer Liliana's Steward Liliana's Talent Liliana's Triumph Liliana, Death Mage Liliana, Death Wielder Liliana, Death's Majesty Liliana, Defiant Necromancer Liliana, Dreadhorde General Liliana, Heretical Healer Liliana, Untouched by Death Liliana, Waker of the Dead Liliana, the Last Hope Liliana, the Necromancer Oath of Liliana
The suite of creature threats seems very powerful, with many cards capable of a short clock without much help