For example:
A powerful sword forged by a famous blacksmith, it has seen endless battles.
A cheap yet versatile mini-tank, the Ice Golem provides a wide range of uses ...
The sister of the Witch, she has moderate hitpoints ...
The capital ship of the Terran fleet, this powerful spacecraft is capable of firing a Yamato cannon ...
That is, a sentence (perhaps following a title) that begins with a descriptive noun phrase, followed by a complete sentence. I remember reading sentences with a similar structure in item descriptions in many video games, but I can't remember a grammatical term to describe such sentences when I want to discuss them with others.