(Dec 2020: Belatedly realized that Unearth is now budget level, and it's a perfect fit for this low CMC deck. Still contemplating targeted hand disruption. Should I worry about planeswalkers?)

Sacrifice engine built around Skirsdag High Priest runs on 2 mana to simultaneously generate 5/5 flying demons, 5/5 giants, pump a vampire, and grow a beetle, with a sideline of discard. Can also cause life loss directly. Very low mana curve, can set up a large board within just a few turns.

Contains plenty of creatures to enable the engine. Creatures and spells in the main deck must be able to punish opponents for using removal, i.e. fulfil at least one of these conditions: recursion, suicidal (self-sac), sac outlet, or have a clear benefit from sacrifice/creature death. Sideboard provides a mix of hand disruption, creature control and graveyard control.

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Main deck:

Unearth: simple recursion X backup cantrip
Cremate: cantrip x graveyard control
Quest for the Gravelord: feeds on creature deaths for a kill con
Tragic Slip: targeted removal, but conditional (not that difficult though)

Bile Urchin, Death Cultist: self-sac chumps that can suicide at instant speed
Blood Artist: trades on sacrifices to ping opponents for extra damage
Bloodthrone Vampire: mana-free sac outlet that can double up as aggro
Mortician Beetle: grows with sacrifice, develops into a pretty large threat
Reassembling Skeleton: sac fodder and chump blocker, instant recursion ability lets the engine roll over and protects it from graveyard destruction
Skirsdag High Priest: main driver of the combo
Viscera Seer: same as Death Cultist but also doubles up as a sac outlet

Sideboard:

Beckon Apparition: graveyard control, instant speed flying chumps
Distress: hand disruption
Geth's Verdict: more creature control that bypasses protection and hexproof while buffing the beetle
Illness in the Ranks: token control, can shut down token decks
Liliana, Heretical Healer: lifelink, token generation, recurring discard, reanimation, overall good for longer games

This deck works best in multiplayer where it can troll everyone else!

NOTE: Liliana and the fetchlands hike the price up. To drop it back down to budget, just switch out the fetchlands for regular lands and replace Liliana with whatever looks suitable from the sideboard or maybeboard.