The Gatewatch has figured out that something's wrong, but haven't quite pieced together what exactly that is
Acererak is a three-mana 5/5 that reads: When Acererak the Archlich enters the battlefield, if you have not completed Tomb of Annihilation, return Acererak the Archlich to its owners hand and venture into the dungeon. It also creates a 2/2 Black Zombie creature when it attacks unless your opponent sacrifices a creature, and it does this for each opponent too
1 Viridian Emissary
Den Protector can dominate a slow metagame, and there are some fun rare morphs from Khans block like Thousand Winds
By combining multiple Runes of Might, Runes of Speed, and Runes of Sustenance onto the same creature, you're going to draw a ton of cards (hopefully more free runes) and swing in for lethal
While you're free to build outside these archetypes, they can be useful for new players who aren't used to Limited