Farsight is a utility spell which greatly increases the PC's vision range for a period of time. The extent of vision range increase is governed by the PC's Mana.
This is a very useful spell which can help the PC in a variety of ways:
If the PC is capable of ranged attacks, Farsight may enable the PC to target and attack monsters from a greater distance than would otherwise be possible.
If the PC uses bolt spells, Farsight may enable the PC to see all tiles that are within the bolt's range. This can ensure that the PC doesn't unintentionally kill certain creatures (such as cats) which might otherwise lurk beyond the PC's vision range and get hit by a long-range bolt.
Farsight reduces the area in which new monsters may appear, since monsters can only spawn on unseen tiles.
Farsight can help the PC to avoid dangerous monsters entirely if they prefer.
The benefits of Farsight also apply while navigating the world map. This can enable the PC to scout the Drakalor Chain for new locations (such as the random dungeons) more quickly.
Farsight can be particularly useful to dark elven PCs in outdoor locations during daytime, since dark elves ordinarily receive a large penalty to their vision range in these circumstances.