Decades have passed since your first planeswalk. As you age, you find your necromantic powers cannot stave off the specter of death forever. You become obsessed with preserving your life. No price is too great, and you seek a contract with four ancient demons.
Liliana needs to defeat all four Demons in succession to gain her demonic contract. As each Demon falls, Liliana will gain its power.
The demons use a single shared deck. When a demon is defeated, the next takes over with its own life total, and Liliana and the opposing demon each draw three cards; the game state otherwise remains the same, including permanents on the battlefield and which part of the turn it is and so forth.
The first demon, Griselbrand, starts with 8 life. The second, Razaketh, starts with 10. The third, Belzenlok, starts with 10. The final demon, Kothophed, starts with 12.
Many of the demons' creatures heavily outclass Liliana's in a straight comparison; on the other hand, Liliana's creatures do a great job matching up against the demons' early ground creatures, and she has some hard removal for those creatures that outclass hers. The basic strategy is to gum up the ground with your ground creatures, deal with the large flyers with removal, and win in the air (with Carrion Crows or by reanimating a large flyer) or by simple attrition (forcing chump blocks, getting your Zombies back with Lord of the Undead, etc.).
The demons have Mind Rot, so you should play around it if possible by holding lands or even an unneeded creature in your hand so you don't discard your important cards like Rise from the Grave or Cruel Revival.
The demons only have two targeted removal options (one Shadowborn Demon and one Murder), so it's hard for them to get back in the game when you get into a good position (such as having a large Soulless One or reanimated flyer). One way to help keep your position is to force chump blocks by attacking in such a way that it's possible for the opposing demon to survive by chump blocking (and only by doing so), so they lose their creatures and you lose little or nothing, even when attacking with everything would result in certain lethal (since then they might block in a way that leaves their creature alive).