Looking at my cube, there is critical mass for: Goblins (R) Humans (all) Warriors (WBR) And, with some effort, other tribes could get critical mass running only cards that are cubeable by themselves, keeping the Poison effect to a minimum: Soldiers (W) Wizards (U) Shamans (RG) Vampires (B) I'm fine with dedicating 2-3 slots per color so each of these tribes have a Lord or some other linear effect as a nod to play them (Goblin Chieftain, Champion of the Parish, Chief of the Edge) but I don't want this cube to be mainly tribal, just have an extra layer of synergy that cares about the creature type line at a low opportunity cost
It also can be used as an event when it's cast, which can be fun, and it's the best way to draw cards in Blue for fun
Anyways, enough rambling about Crook of Condemnationlet's move on to our second card: Solemnity
Dread Wanderer will be a nice role-player for multiple archetypes
You would get very powerful effects at an efficient mana cost in exchange for giving your opponent a fair warning and the chance to play around the telegraphed effect