According to
this, it's "90% down", which in Japanese probably means "down to 90% of original value", or "10% down" in English.
The same page also says due to long cast time, low accuracy, low proc rate on additional effect (that would be attack down), very few people like to use it.
Edit:
If I'm reading this right, when ghosts use Terror Touch, it's a "15% down"--that would be attack down 85% in English. o_O;