You may cast legendary spells and artifact spells as though they had flash.
If a legendary permanent or an artifact entering or leaving the battlefield causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.
Gandalf the White is a mono-white flash deck that doubles every legendary/artifact enter-or-leave trigger you control while letting you deploy those permanents at instant speed. You hold up mana, cast key legendaries and artifacts on opponents' turns, and snowball value through doubled ETB/LTB triggers until you grind the table out. The deck plays like a reactive value engine that punishes opponents for tapping out.
Doubles a huge swath of triggers (ETB, LTB, artifact-flicker payoffs) for explosive value
Flash on legendaries and artifacts gives strong instant-speed flexibility and information advantage
Mono-white access to deep artifact and legendary support plus strong recursion
Can ambush blockers, flash in answers, and play around removal effectively
Mono-white struggles with raw card draw and ramp compared to other colors
Heavily commander-dependent; loses the doubling engine if Gandalf is repeatedly removed
Lacks reliable interaction outside of white removal and artifacts
Trigger-doubling does nothing without enough enter/leave engines online