A similar entry appears in the Wheel of Time Companion confirming the information available in the main story arc.
Ashandarei literally means "sword spear" in the Old Tongue (ACoS, Ch. 37), which is what it looks like: a long, black-hafted spear with a foot-long "sword blade in place of a spear point, slightly curved and single-edged". They are not in common use, but Mat knew from his memories something of the way to fight with one.(TSR, Ch. 37) They are a two handed weapon (WH, Ch. 30), best used with open space (ACoS, Ch. 38) and are infective without room to move them. They function as wither a quarterstaff or spear.
Mat has one that was given to him by the Eelfinn and has inscribed on it, on the Old Tongue:
Thus is our treaty written; thus is agreement made.
Thought is the arrow of time; memory never fades.
What was asked is given. The price is paid.
The blade is power wrought and the staff a black wood. There are two ravens on the blade, and one at either end of the staff, in a metal darker than the wood.
Mat's Ashandarei turns out to be the answer to Mat's wish to have a way away from the Finns. He uses it to open a doorway to escape the Aelfinn when rescuing Moiraine from the Tower of Ghenjei by carving a triangle with a wavy line through it into the wall.(ToM, Ch. 55)