Quiver

Quiver (released in 1997) is a notable shareware first-person shooter created by ADvertainment Software and published by ESD Software. Arriving near the end of the commercial MS-DOS gaming era, it was a 2.5D FPS built to compete in the wake of Doom, Heretic, and Duke Nukem 3D.
You play as an operative sent on a mission through time and space. An alien race known as the Gargoyles has invaded Earth and stolen a collection of powerful orb-like artifacts known as Orbs of Power. They have scattered these orbs across different historical time periods to disrupt human history.
Your objective is to travel through various time zones (including medieval castles, ancient temples, futuristic bases, and alien worlds), retrieve the stolen orbs, eliminate the alien forces, and prevent the collapse of Earth's timeline.
Unlike many early shooters where the goal was simply finding a red/blue/yellow keycard to reach the exit, Quiver required you to locate a set number of Orbs hidden throughout each level before the exit portal would unlock.
The game featured a varied arsenal ranging from standard projectile weapons to energy cannons and alien tech. It also included inventory items and power-ups like flight/levitation, invulnerability, and speed boosts.
The game followed a shareware distribution model split into three main episodes:
Episode 1: The Gathering
Episode 2: The Red Moon
Episode 3: The Dark Citadel (Registered Version)

What is abandonware ?