7 mana is really expensive for a commander, however, and 6 isnt much better from Phylath, so Id recommend playing a lot of ramp and card draw if you elect to build decks around these legendary creatures
The other three rocks are staples
Mu Yanling, Pathfinder (Showcase Fracture Foil) Another one with gorgeous art, I wouldn't be surprised to see this slotted into a blue artifact-focused Commander deck like Urza, High Lord Artificer
The best part about another round of precons is the amazing variety on offer - between Faceless Menace's morph, Mystic Intellect's flashback, Primal Genesis' populate and Merciless Rage's madness, these decks are likely to have a play style for everyone
That said, we already have a pretty powerful black discard deck in Standard, with cards like Liliana of the Veil and Bandit's Talent
Are you picking up either or both of these decks