Descent 3: Mercenary is a seven level expansion pack to Descent 3, it was released in 1999 six month after the original game. The games story takes place inbetween the end of Descent II and the first few missions of Descent 3. The player controls this time around not the Material Defender, like in other Descent parts, but a mercenary that is involved in attack and sabotage missions, some of those attacks triggering events happening in the original Descent 3 plot.
The core gameplay of Mercenary is identical to that of Descent 3, new weapons or game objects are not provided as far as I can tell, aside from a few new enemy types. Descent 3 problems are also untouched. The guide bot still moves rather erratically in large environments, which makes it extremely hard to follow, it can also still get lost from time to time and not find the way back, both problems didn't exist in Descent II. Weapon and enemy fire still lack a proper feel for impact, making it hard to judge if you hit something or got hit. Enemy design is also still rather problematic, with enemies shooting to many to fast moving projectiles that make it impossible to properly dodge them other then by wild circle strafing. The game also again contains plenty of unbalanced gameplay situations, where you run into spots one is attacked from multiple sides by multiple enemies, making it hard or impossible to find a save spot. Enemies also reduce your health rather quickly, sometimes in two hits, which often leads to random death before you even know what was going on.
The reset point system from Descent 3 is present again. It will respawn you with your weapons gone and placed on your death spot for recollection without reseting the game world, thus enemies will stay exactly where they where when you died. This keeps frustration rather low, even in unfair situations. The game does however have a few weird spots where you have to do something under a timelimit and failing to do so will result in a failure of the mission, with the player being reset to the start of the level. Thus saving before pressing a switch that might trigger a time limit is needed to avoid running into situations that force a replay of a level.
As with the original game, Descent 3: Mercenary is again rather heavy on the puzzles. A lot of times you will have to press a switch or perform other puzzly acts and not be much involved in combat. Combat takes more of a side role, it's there, but it often feels more like a small obstacle between the current puzzle and the next, not the core focus of the game. The reset system, which often lead to a simple death/retry loop further enhances that feeling, as being actually good in combat provides little if any benefit. The game also introduces a few weird "soccer puzzles", in which you have to push a ball around and navigate it to a specific target, those feel kind of even more out of place then the regular switches. One of those puzzles also felt rather broken, or well, maybe I just couldn't figure out the proper solution. All it involved was kicking a ball through a pipe, except that none of the weapons seemed to have any effect and the only way to push it was with the ship itself. Touching the ball however lead to instant death, no matter how slow and gentle. I solved the puzzle simply by crash, respawn, crash, etc. On the positive side of things I however found none of the puzzles to be as complicated and time consuming as some of the bad ones in the original Desecent 3.
Overall Descent 3: Mercenary is simply more Descent 3, with all the problems and issues still intact. The thing I like about Descent is the fast navigation through narrow tunnels and the way you can manoveur around enemy projectiles with the 6DOF control scheme. In Descent 3 and this add-on, however all the locations are large, making the navigation feel slow and boring and the number and speed of enemy projectiles makes them near impossible to dodge individually and you quickly resort to random circle strafing and random death. The focus on puzzles also just feels out of place. The last boss fight, in the tradition of the series, also feels incredible unfair again and the ending is, unlike the fully animated intro, just a disappointing still frame with a lazy "The End" text, the add-on doesn't even bother to scroll any credits. So while this isn't a bad game, it again fails to reach the real potential that I see in the Descent series.