Timeless Arrow

Timeless Arrows were the most powerful type of arrows available in, adding 53 DPS (not counting Nether Spike that can only be used in one battle, and would then be considered the second best.)

Source
These arrows were sold by Andormu and Nozari  in Caverns of Time for. You had to be with the The Scale of the Sands to purchase these arrows.

There are also ammunition vendors on both Scryer Tier and Aldor Rise occupying a previously uninhabited building on the right side of each camp (as viewed from the Terrace of Light). These vendors sell all ammunition from Blackflight Arrows and up (with the exception of the Halaa ammunition and the crafted Adamantite Stingers). You must have the required reputation with their respective sources to buy a given ammunition type from these vendors.