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Here are some cards similar to Ureni, the Song Unending (assuming you mean a blue/white card focused on legendary spells and cost reduction):
1. Jhoira, Weatherlight Captain – Draws cards when you cast historic spells (including legendaries).
2. Kykar, Wind's Fury – Is Jeskai, but makes tokens when casting noncreature spells (some overlap).
3. Kethis, the Hidden Hand (white/black/green, not matching colors, but legendary spell cost reduction).
4. Raff Capashen, Ship’s Mage – Lets you cast historic spells (including legendary) as though they had flash.
5. Narset, Parter of Veils – Not exactly cost reduction, but is Azorius and influences card draw.
Closest equivalents for legendary matters and being blue/white are Raff Capashen, Ship’s Mage and Jhoira, Weatherlight Captain.
Reason: Both emphasize historic/legendary spell interactions and share at least one color identity.
