Like so many decks, this one started with a card from the random card generator at Scryfall (https://scryfall.com/random): [[Deathknell Berserker]]

Play creatures and keep them away from your graveyard by providing them with quasi-undying. Two of the creatures in this deck gain you an advantage when dying, the rest simply likes to not die. Some gain you additional benefits, while others are only an annoyance for your opponent. With 10 undying spells in the deck you should always be able to stop an important creature of yours from dying.