in cEDH, Blood Pet is often found in Krrik, Son of Yawgmoth builds as a way to net an extra mana for 2 life, but also sees a lot of play in Inalla, Archmage Ritualist and Ad Nauseam lines to rush out these expensive spells or aid in comboing off with other cards to win the game
The last is the nasty one that puts this deck over the top as a life-gain trigger for combos that you can never stop
Regeneration cannot save the creature with toughness 0 or less
By proliferating the counters on Agents ToolkitAgents Toolkit, they can make extra counters and move those extras off to make the Toolkit act more like a Bag of Holding from Dungeons and Dragons
Lore-wise this is actually a wizard who's fallen victim to mana burn, a now defunct rule in Magic which made players' own unspent mana deal damage to them
With more players, special deck construction, and an additional way to lose, it can be difficult to think of a deck to build