When Lavinia enters, detain each nonland permanent your opponents control with mana value 4 or less. (Until your next turn, those permanents can't attack or block and their activated abilities can't be activated.)
Lavinia of the Tenth is a control/stax commander who freezes opposing boards on entry, then grinds out the game with card advantage and a clean attack step. You play defensively early with removal and ramp, drop Lavinia to detain their cheap permanents, then flicker or recast her repeatedly to keep opponents locked out while you build toward a finisher.
On-cast detain neutralizes multiple cheap permanents at once, buying huge tempo against go-wide and mana-dork strategies
Protection from red makes her durable against burn, red-based removal, and red attackers
UW has the best control suite in the format—counterspells, board wipes, and efficient removal
Flicker and recursion turn her ETB into a repeatable lockdown engine
Detain is a one-shot, temporary effect that does nothing against permanents with mana value 5+ or future plays
Five mana for a 4/4 with no evasion is slow and clunky as a beater
Vulnerable to non-red removal, sacrifice effects, and bounce that reset her ETB value only when you can replay her
Lacks inherent card advantage—you must supply the engine yourself
Recurs Lavinia and other low-cost stax and value permanents from the graveyard.
Lean harder into repeatable flicker (Brago, Conjurer's Closet, Ephemerate) so detain happens every turn, and add stronger control density—counterspells, Smothering Tithe, and Drannith Magistrate—to deny opposing combos. Add efficient win conditions like Approach of the Second Sun or a compact token finisher so the lock actually closes games, and improve mana with fast rocks and Land Tax-style card advantage.