Ongoing discussion of cards I think are underrated in modern.
My contention: Liliana's Mastery is one of the best support cards to Zombies that's been printed in quite a while, and in general represents exactly what mtg wants from tribal mechanics.
Normally, I would start by giving a brief description about what pros and cons the card has, but since this is a tribal card, its true relevance lies within its tribal mechanic, and the tribal mechanic in general, so that's where I'll start.
Tribal in mtg is all about the creation of synergy, or the idea that 1 +1 = 3 in specific circumstances. Most tribals do this by either generating superior aggro status, superior card advantage, superior control, or superior tempo (or combos of the above), thus making traditional notions of any of the above less effective against them.
For example, if you have 3 zombies and opponent has 3 non-tribal creatures, your wager by playing zombies is that together your 3 zombies will generate more advantage than a set of 3 non-connected creatures, putting you ahead enough to justify the choice to play zombies. In general, this advantage is achieved by either spawning superior numbers, or with lord effects. Lord effects are buff ups given to all tribal members, usually from a creature costing 3 (or 2 if your lucky and a merfolk).
Different tribals focus on slightly different generated advantage: merfolk control and buff, goblins generate crazy tempo, etc., but basically all of them want 2 things in particular- superior numbers and a lord effect.
Liliana's Mastery does both, and with surprising and potent efficiency. This 1 card spawns 2 tribal members, and has a decent lord effect, all for an affordable amount of mana. What's more, all this advantage is generated into 3 distinct entities (1 enchantment, 2 tokens) making it almost impossible to not retain some sort of card advantage from it as opponent will need multiple cards/and or/card types to remove every threat generated by Mastery. Spawning 2 tokens means a single removal spell won't get both of them. The independent lord effect of the enchantment means opponent would have to have a destroy enchantment effect (barring a generic destroy permanent effect), and in order to get all 3 entities generated by Mastery, opponent would have to run one of those rare "destroy all permanent" effects, which is extremely unlikely.
In addition, Mastery does EXACTLY what you want from a zombie mechanic. Zombies love to swarm and lord each other, and a turn 5 or 6 Mastery could and probably does mean an attack from 3 3/3's or 4/4's with the threat of more to come next turn.
On the negative side, most of the advantage it generates, the tokens, are vulnerable to "token removal" which is heavily sided (engineered explosives, etc). But, as stated above, even if they run token hate, you still retain the lord effect and force them to drop a card to nuke your tokens, netting you the +1 on the exchange. Also, vulnerability to token hate is realistically an unavoidable consequence of many tribal mechanics, so the decision to avoid it is silly.
Conclusion: Liliana's Mastery is a must consider in zombie decks due to the lord effect, swarm mechanic, excellent card advantage, and perfect fit for the zombie mechanic in general. And honestly, its not even that bad in a deck that doesn't run zombies, as a 5 mana card nets you 2 3/3's.