If "alight" means "to descend and settle" (like a bird landing), then the antonym would be:
* Take off
* Depart
* Ascend
* Fly away
If "alight" means "to be lit or set on fire", then the antonym would be:
* Extinguished
* Unlit
* Dark
* Put out
If "alight" means "to be happy or excited", then the antonym would be:
* Sad
* Depressed
* Disheartened
* Unenthusiastic
To provide a more precise antonym, please provide the context in which you are using the word "alight".