If anyone else attempts to use the sword, many of its special powers are lost.
It is a large, glowing golden sword which gives off constant light.
Special powers resulting from possession of the sword include a huge increase in Combat Skill, extra damage to Undead and ability to harm a number of evil creatures immune to normal weapons, including Helghast and (crucially) Darklords. It is sometimes described as the only weapon capable of slaying a Darklord, although in some battles in the books this is patently untrue (since in Lone Wolf 12, the Darklord Gnaag can be harmed by zenjet-dulaaga and other magic weapons). At the end of Lone Wolf 2 and the novel version (Legends of Lone Wolf 3, or 4 in America), Lone Wolf lets loose a burst of sunlight channelled through the Sommerswerd, instantly slaying the Darklord Zagarna and putting to flight the army besieging the Sommlending capital, Holmgard.
The light emitted by the sword proves a liability in Lone Wolf 12: The Masters of Darkness, when Lone Wolf must travel into the Darklands themselves and slay the Darklord leader (the third in succession!), Gnaag. The light would draw enemies to him, so he has to keep the sword sheathed until he meets Gnaag himself, in a special scabbard prepared by the Elder Magi.
Less well-documented is the fact that the final battle of Lone Wolf 8: The Cauldron of Terror becomes virtually unwinnable if Lone Wolf is equipped with the Sommerswerd, whereas it is quite winnable with any other well-equipped character. (Fortunately, Lone Wolf can already leave the sword in safe-keeping at the Monastery if a player foresees this eventuality).
Additional powers appearing only in the Legends series include the production of an imaginary being able to appear only to the Kai Lord wielding the sword, who channels the wisdom of the sword's makers directly to this individual.
The name means "sword of the sun" in the invented language Sommlending (which seems to be based on old English).
If you are using the Sommerswerd, double the damage received by this opponent.
Lone Wolf was sent on a mighty quest to retrieve the Sommerswerd in Sommerlund's time of need.