In grindy matchups, Bloodthirsty Conqueror can put in a lot of work since it has decent stats, Flying, and Deathtouch
You can choose to either: Have an opponent lose life equal to the amount of life they already lost this turn You gain life equal to the amount of life you gained this turn Either one of these abilities are pretty darn good, and with the amount of lifegain and damage Vampires can dish out, you'll likely be triggering the effect every turn
Way back in the day, I had a mono-black Relentless Rats Commander deck, with Marrow-Gnawer at the helm
Illuminor Szeras Its possible to make some silly amounts of black mana with Illuminor Szeras, but you only ever get back the same amount of mana you put in unless you make your creatures cost less
Through pinging for just one or two damage each turn, you end up with a fistful of cards and enough life to survive being hit back
The one loss here was not being able to tap into old legendary nostalgia, but Commander Legends is doing that later on in the year so well that preserving those throwbacks for that set worked out