Havoc Festival

Players can't gain life.
At the beginning of each player's upkeep, that player loses half their life, rounded up.

Bloodcrazed Socialite

Menace
When Bloodcrazed Socialite enters the battlefield, create a Blood token. (It's an artifact with "{1}, {T}, Discard a card, Sacrifice this artifact: Draw a card.")
Whenever Bloodcrazed Socialite attacks, you may sacrifice a Blood token. If you do, it gets +2/+2 until end of turn.

Olivia, Crimson Bride

Flying, haste
Whenever Olivia, Crimson Bride attacks, return target creature card from your graveyard to the battlefield tapped and attacking. It gains "When you don't control a legendary Vampire, exile this creature."

Hurl Through Hell

Exile target creature. Until the end of your next turn, you may cast that card and you may spend mana as though it were mana of any color to cast that spell.

Kamber, the Plunderer

Partner with Laurine, the Diversion (When this creature enters the battlefield, target player may put Laurine into their hand from their library, then shuffle.)
Lifelink
Whenever a creature an opponent controls dies, you gain 1 life and create a Blood token. (It's an artifact with "{1}, {T}, Discard a card, Sacrifice this artifact: Draw a card.")