Carl's bandana, which grants the Tripper spell at level 15 with a five-hour cooldown instead of ten minutes.
The kerchief is the source of Carl's level 15 Tripper spell, named for Drakea. Its version of the spell is far stronger than Carl's innate one but carries a five-hour cooldown rather than the spell's ten minutes, and the two cooldowns cannot be mixed and matched — a distinction Mordecai walked Carl through at exhausting length. Removing the bandana revealed Carl's own Tripper had independently risen to level 7.
The original plan for the naga castle raid was simply to lend the kerchief to Imani or Elle. It failed: fairy-class and other magical races turn out to be extremely limited in the sort of magical gear they can wear, something Carl had not known. That restriction is what forced the party to the Blood Tattoos workaround instead.
A patterned bandana Carl wears knotted over his head.
It is the item source of Carl's Tripper spell and the reason the tattoo workaround was needed.
Also appears in books you've already read — spoiler-safe to their endings: