Quote from Gutterstorm »I did a similar thing many years ago going from Zegana to Vorosh(Don't judge me at the time he was almost the only Sultai commander in existence). The biggest thing black gives you is Corpsejack Menace and good removal. Other than that black's not going to give you much unless you want to get some -1/-1 counters and infect going.
Notably Volrath can transform off of any counter so Tetzimoc, Primal Death seems like a solid choice. A source of counters and removal.
Your lack of Champion of Lambholdt disturbs me. Look out for Loyal Guardian as well. Gonna be solid dropping counters on all your dudes.