Other legendary creatures you control have vigilance and ": Add one mana of any color."
At the beginning of your upkeep, reveal cards from the top of your library until you reveal a creature or planeswalker card. Put that card onto the battlefield and the rest on the bottom of your library in a random order.
Cast The Prismatic Bridge side and use its upkeep trigger to cheat huge creatures and planeswalkers onto the battlefield for free, snowballing into an overwhelming board. Alternatively, cast Esika to enable a five-color legendary tribal ramp deck that turns every legend into a mana dork. Either way you assemble a critical mass of powerful bodies faster than fair decks can keep up.
Free top-deck cheats out a creature or planeswalker every single upkeep, generating relentless card and tempo advantage
Full WUBRG identity means access to the best removal, ramp, tutors, and bombs in the format
Esika's mana ability turns legendary creatures into rainbow mana sources, fixing and accelerating any plan
Two strong modes on one card give flexibility and resilience to the command zone
The Prismatic Bridge whiffs on lands or can hit weak hits if your creature/planeswalker count is low
Vulnerable to enchantment removal and board wipes that undo the value engine in one card
Bridge costs five mana and does nothing the turn it lands, so it's slow against fast combo
Stacking legends with Esika invites Pithing Needle and graveyard/removal hate
Massive payoffs the Bridge can cheat in for free, even with no cast trigger.
Add fast ramp (Sol Ring, Arcane Signet, mana dorks) and tutors (Demonic Tutor, Eladamri's Call) to deploy the Bridge on turn three or four. Optimize your nonland top-deck density so the Bridge never whiffs, leaning on a high count of impactful creatures and planeswalkers over situational spells. For a higher-power finish, include Tooth and Nail combo packages and protection like Heroic Intervention and counterspells to defend your engine.