The Ancient staff is one of the only items with a negative prayer bonus, along with Zanik's crossbow and the Amulet of zealots.
Critical hitsplats do not appear with the ancient staff as they normally would with most other attacking equipment. Whereas most other equipment (such as a Slayer Helmet) used to give a critical hitsplat every time the attack goes over the damage it would normally do, when using an Ancient staff, the critical hitsplat will not appear unless the maximum damage with the bonus is achieved, such as a hit of 220 with Fire Wave.
During the quest Dream Mentor it is referred to as 'a staff of Zaros' when looking into Cyrisus' chest.
It looks extremely similar to the Ancient crozier.