I love merfolk, of course you are only playing 1 of the good merfolk. You are missing the 2 best lords (Master of the Pearl Trident and Lord of Atlantis)
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Just go ham on the combo but maybe add more ramp. You don't need door, another Stone and 3 more rats would be better. Actually this might be better in legacy with Dark/Cabal ritual.
Suggestions:1. Fetches (of course)2. Rally the ancestors is actually really good in this deck. 3 mana to bring back all your 1 mana sac creatures. Play four.3. Cut dictate for more board wipesOverall advice: Play this more like a combo deck.
Reminds me of my legacy deck, same concept except you ritual, ritual, undercity informer, mill deck, play 4 narcomeaba, dread return, Angel of Glory's Rise, draw a card. No lands in the whole deck.
With the Hexmages you don't really need downfall (the only really prevalent 'walker is lili anyway) so you should probably run more terminates instead.Nether Traitor is also solid in this deck as a 2 drop as it also reanimates and has shadow, also it's reanimation is based on creature dying.You are actually pretty light on red and the only red creatures are 4 drops, more swamps might be better