There have been a lot of Morphling variants over the years, most of which have fallen flat
Alongside Kitchen FinksKitchen Finks, which finished outside this top 10 but is still a great card, Redcap also made my namesake Nicholas Lucchesi's best hybrid mana cards in Commander list
Also, Olivia Voldaren is red in addition to black, and Threaten-effects are a mainstay there, it's Captivating Vampire that's the rare "flavor over color pie"-effect we're talking about
Sacrifice after III.) I - Gungnir - Destroy target creature an opponent controls
Passively creating a flying creature every turn is a fantastic way to beat board wipes and put some pressure on planeswalkers
You can either go heavy zombies and have the drain sac plan as a backup or your can just play Belcher in a zombie light Blood Artist deck and have it be a good all round card that adds a little punch and utility to the mix