lengthy (esp. BrE), long, prolonged
brief, short
five-minute, ten-day, etc. (esp. BrE)
good
bad
He's going through a bit of a bad spell at the moment.

quiet
barren, lean (both esp. BrE)
Viera ended his barren spell with a goal against Parma.

cold, dry, hot, mild, sunny (esp. BrE), warm, wet
dizzy, fainting
The children began having dizzy spells from hunger.

go through, have

during a … spell
She managed to write a letter during a quiet spell at work.

spell as (esp. BrE)
He had a brief spell as ambassador to Turkey.

a spell of … weather (esp. BrE)
a spell of sunny weather

magic, magical
powerful
hypnotic
the hypnotic spell of the cicadas singing in the trees

be under, come under, fall under
cast, perform, put, weave
The witch cast a spell on them.
This place had woven its spell over them.

break, remove
She uttered the magic word, and the spell was broken.

book

under a/the spell
Sleeping Beauty was under a spell when the prince found her.

under sb's spell
The audience was completely under his spell.