While all the legendary creatures well be looking at have black as part of their color identity, I am going to cover a spread of different color combinations so you can choose the one best suited to your goals and playstyle
You can then tap it and remove it the counter on it to add the exact amount of mana you spent to attach the counter and card in the first place, and then that mana can only be used to cast the card attached to Ice Cauldron
Another angle is to play with some of those suspend cards with no mana cost, like Ancestral Vision, Crashing Footfalls, Resurgent Belief, Lotus Bloom, or Mox Tantalite, and get a zero-mana creature in the graveyard for Ellie and Alan to discover them with
They're kind of like number words
Think about that against something like Wailord TR thats 170 damage for one Energy
A huge creature with reach, trample, and cascade is certainly worth paying 7 mana for even if its countered