Terminus Est refers to the flat-bladed executioner's sword belonging to Severian, the protagonist of the Science Fiction/Fantasy series called The Book Of The New Sun. Severian is a member of the torturer's guild, and is expelled from it for showing mercy to a prisoner, Thecla.

(Thecla, who may or may not have been Severian's lover, was run through a machine called The Revolutionary, which creates a self-destructive alter-ego in its victim - they literally tear themselves apart with their hands when they are not concentrating on suppressing this subverted personality. Severian gives Thecla a knife, allowing her to make a quick end of it.)

Instead of being executed himself, he is sent out of the guild to make his living as an executioner. Before he leaves, in a gesture of sympathy, one of his former masters (the aforementioned Master Palaemon) gives him a sword to use for this trade - Terminus Est. The blade is filled with mercury (hydrargyrum is the specific word used) in a hollow chamber in the center so that it can carry more force when dealing a blow - an executioner should never have to strike twice, for obvious reasons. As it is not used to stab, Terminus Est does not taper to a point, but is instead squared off at the end. Because it is not supposed to be used to parry, Severian keeps the edge razor-sharp, and the narration describes him whetting the blade and testing it by splitting a hair on its edge.

Also, the name of a poisoned executioner's sword
in the game Castlevania: Symphony of the Night.
The icon has a wee adorable squared-off end.