Centaur Blood

Centaur Blood is the blood of a centaur. It is highly corrosive of flesh (and just about anything) like acid.

The Titan's Curse
In The Titan's Curse, Travis and Connor Stoll give Phoebe a t-shirt with centaur blood on it. This causes her to break out into hives, and is unable to go on the quest.

Zoe Nightshade, a freind of Phoebe says centaur blood is like acid "Centuar blood is like acid. Everyone knows that."

The Battle of the Labyrinth
Centaur blood was used in the traps they used against the Titan army near the end of the book.