Furfur is a powerful Great Earl of Hell, being the ruler of twenty-nine legions of demons. They are a liar unless compelled to enter a magic triangle where they give true answers to every question, speaking with a rough voice. Furfur causes love between a man and a woman, creates storms, tempests, thunder, lightning, and blasts, and teaches on secret and divine things.

Commission for  BlackAntlers

The statement entering and leaving the triangle is a corruption of "ab uno disce omnes", which translates to "from one, learn all" (a single observation indicates a general/universal truth)