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It's Luden's. Luden's proc damage is dealt by a missile which has a short delay (even for the primary target) before the missile is fired + damage is dealt. As such, Luden's deals its damage with a short delay, which varies based on the number of targets being hit (slightly larger delay for fewer targets so that the missiles don't overlap).
I think the part that makes this hard to interpret in spectator is that we cull or minimize some fx in spectator for clarity purposes (and in fact many vfx are slightly different for allies/enemies/self/spectator in-game for a variety of reasons, including clarity). Luden's, I believe, is one of the items that has reduced (or maybe culled?) vfx in spectator because there's not normally much value in spectators having precise information of when exactly Luden's is damaging a target.
Luden's does also have some logic to determine if the hit vfx should play if the target dies while the missile is in-flight, which might have broken here for one reason or another (it might just drop the vfx if the target dies and it's not a champion since that's low-priority - I don't remember). It's been a while since I've opened the Luden's logic so it could be either one.
Either way, I'm reasonably confident that this Elder got put to 2hp through Nunu's smite + some other damage (maybe a jungle pet) and then Vex's Luden's damage hits it while it's at 2hp and kills it.