Mono U could likely support a rare mill deck (Ruin Crab, Maddening Cacophony, Field Research, Glacial Grasp, blockers like Skyclave Squid to fill the curve, etc), though most Mono-Blue inclined decks will likely just end up UB
If youre wondering what the best rare cards in the game are, we scoured the Pokedex to get them all
It's literally an Unclaimed Territory 99.99% of the time as spending 6(!) total mana to make a 1/1 Sliver token is such an awful rate that it might as well not even be on the card -- if you regularly activate it then I'd recommend rebuilding the deck, maybe add some more card draw so you have better things to spend your mana on
If you want a powerful, unique, and pretty budget five-color commander, Urtet is still my number one option for you
MTG Arena Legality: The Lost Caverns of Ixalan (LCI): Standard, Explorer, Alchemy, and Historic The Lost Caverns of Ixalan Commander (LCC): Not available on MTG Arena