Nayas identity on Alara was one of large creatures: Many cards provided extra value for controlling a creature with power 5 or greater
Themberchaud isnt cheap, so alongside making sure you hit all of your land drops with card draw, youre going to want to run Dracogenesis to cast it for free, Hellkite Courser to haste it out for a turn, and cards like Mana Geyser for a big turn to burn people out
Bumbleflower does give you some counters and evasion in exchange
Mire Triton One of only three mono-black creatures among Magic's merfolk, Mire Triton is a fine self-mill enabler
There are so many great black cards that take some life, which you can offset with removal that provides lifegain
Lord of the Unreal is an Illusion lord that rarely sees play outside of Changeling decks, but it will fit beautifully here