The party is fighting a masked assassin, and the wizard, wanting to question the assassin, casts Eyebite and attempts to use the Asleep effect. However, the assassin was an elf, and is thus immune to being put to sleep. The next round, the wizard attempts to use the Sickened effect of the spell on the same assassin. Can they do this? The spell states (Player's Handbook p.238):
You […] can't target a creature again if it has succeeded on a saving throw against this casting of Eyebite.
While the assassin did not roll anything, they did successfully avoid a casting of the spell. Does this count as succeeding on a saving throw without a roll, or is the immunity something different?
Best Answer
They should have been forced to roll the initial save regardless
The wizard cast Eyebite, which says (emphasis mine):
The wizard selected Asleep. At this point the assassin rolls a Wisdom save, as directed. If the assassin makes the initial save, they are not affected the current round, and cannot be a future target because the wizard:
However, if the assassin failed the initial save, then the Asleep effect is applied to them. At that point it has no result because of their fey ancestry.
See also: Does a creature that is immune to all outcomes (conditions, damage, etc.) of an effect still make a saving throw? and Does Calm Emotions still allow a saving throw?
Now the assassin is under the effects of the Eyebite spell, but the specific asleep effect is 'suppressed'. If something removed their fey ancestry (for example, polymorph) then the asleep would take effect. The spell has not ended, since its duration is Concentration / 1 minute, and the suppressed effect will persist until the end of the spell.
If the wizard then wishes to try Sickened on the next round, they may.
The 'on each of your turns' says you can target 'another' creature, which by itself might imply that you can't target the same creature again. However, by then saying you can't target the same creature again if it succeeded on its save, the spell clarifies that you can re-target the same creature if it failed.
When the wizard again targets the assassin:
So the assassin now gets a new, second save. Success on this will mean that they are not sickened and cannot be targeted again. Failure will mean that they are sickened. But in either case, success or failure, they will still be carrying the suppressed asleep effect until the end of the spell's duration.