Izzet Delver is a fast tempo-aggro deck built around Dragon's Rage Channeler and Murktide Regent. Murktide Regent costs {5}{U}{U}, so it is a blue spell even when delve reduces its cost. Consign to Memory only counters triggered abilities or colorless spells, so it cannot answer Murktide and has too few good targets here, which is why it stays out. Dauthi Voidwalker is the better fit. While it is on the battlefield, cards that would go to the opponent's graveyard are exiled with void counters instead. That starves Dragon's Rage Channeler's delirium and takes away delve fuel for Murktide Regent. Its shadow also makes it a hard-to-block clock. The extra Fatal Push copies answer Dragon's Rage Channeler cheaply before it grows with delirium, which is important since their clock is fast. Mockingbird isn't a great copy target against this matchup's small, efficient threats, and Wail of the Forgotten's slower value modes are less relevant than cheap, efficient answers against an aggressive deck. Keep Daze and Force of Will in since countering their threats or removal on the cheap is essential to not falling behind on tempo.