Something that's deathly reminds you of death. If you dress up as a ghost for Halloween, the makeup on your cheeks might turn your face a deathy shade of gray.
A spooky silence can be described as deathly silent, and an unhealthy skin color is often called a deathly pallor. When you shiver, your friend might take your hand and say, "You have a deathly chill." An older meaning of deathly, used especially in literature, was "causing death." The Old English root word is deaþlic, "mortal."