"You found a Wisp! Wisps are what remains of past Guardian Spirits. This guardian spirit lived for X hours and X minutes, but died trying to protect Nibel."
Wisps are a special type of collectible exclusive to One-Life Mode. If a One-Life Mode file is restarted after death, a Wisp will mark the place upon which Ori died. Collecting a wisp will award Ori with a large amount of Spirit Light.
It is not advised to try and retrieve Wisps from dangerous places (e.g., directly above spikes) at the risk of a progressed game in One-Life Mode.