The Beo Sword (ベオの剣 Beo no ken) is a special Sword that is unique to Fire Emblem: Thracia 776. This sword can strangely only be used by Diarmuid and Fergus, as Fergus is widely rumored to be the illegitimate child of the legendary mercenary knight Beowolf, while Diarmuid's reason is unknown, causing a popular fan-theory that the two are half-siblings. When equipped, it grants the Wrath and Ambush skills to its wielder, which are skills that Beowolf cannot use.
* - Usable by Diarmuid and Fergus only.
The "Beo" portion of the sword's name could possibly be derived from Beowolf's name, further hinted at by his alleged children, Diarmuid and Fergus, being able to wield it. An alternate translation of the name could be "Beo's Sword", further implying, at the least, that is was owned by someone with the name or a name similar to "Beo".
Alternatively, the name of the sword could take its origins from the Irish verb "beo", that which directly translates to "live".