The incorrectly spelled word is:

☀ Sieze

The correct spelling is "Seize". It means to take hold of forcibly or to capture.

Explanation of the other options:

☀ Grief: This word is spelled correctly. It refers to intense sorrow or sadness.

☀ Piece: This word is spelled correctly. It refers to a portion or part of something.

☀ Fierce: This word is spelled correctly. It means showing a strong or aggressive nature.