A game ender is a unit which when built effectively ends the game. This is generally due to the fact that all game enders are so insanely powerful they have no real countermeasures. This is of course balanced by their immense costs. Most of them are experimental units.
Examples
Mavor - An artillery piece with near infinite range, exceptional accuracy and extreme damage. While the Mavor does not excel at laying waste to entire bases, the fact that its shots go through shields, and are powerful enough to destroy almost any building in one shot is enough to end the game. It can be used to strategically destroy every opposing enemy power generator, mass extractor, or any other strategic building type. Destroying SMDs to pave the way for nuclear attacks in addition to the Mavor is not a bad idea at all. Firing at the enemy ACU is also very effective
Template:-
Yolona Oss - Fires a massive nuke requiring two anti nukes to stop, with a very fast nuke build time. The real power of this weapon doesn't come from its obscene damage (1,000,001), but from both its fire rate, one nuke every minute, and from their price, almost free, compared to a conventional Nuke Launcher, where it takes eight minutes to fire, and the nuke costs as much as an experimental unit. It could be countered if the enemy was prepared, although it will wear them down over time. The opponent has to build eight anti nukes working full time to meet the Yolona Oss' production, which unless they are an Aeon player who already has multiple Paragons will most likely crash their economy.
Template:-
Paragon - Near Infinite resources. Once this is built, you can start mass producing experimental units.
Template:-
Salvation - The only non-experimental game ender. A very long range artillery installation, which allies extreme DPS and damage radius. Can flatten entire bases quickly, or destroy any ground forces (including experimental units), approaching it.
Template:-
Scathis - The only mobile game ender. It achieved its game-ender status in Forged Alliance, gaining the range equivalent to a Mavor. It is not nearly as accurate as a Mavor, but fires far more rapidly, its shots have a wide area of effect, and it can move from place to place if needed.
Notes
Whilst these units are effective game enders, there are many other ways to end a game, such as several nuke launchers, firing at one area. Or several T3 artillery. Although if you can afford these without your opponent countering, you could have won in several other ways by time everything is ready. To put things in perspective, if you have enough resources to build one of the units above, you could also have built about 20 Monkeylords,18 Fatboys, 15 Ythothas or 15 Galactic Colossi. In addition, one Mavor is equal in price to exactly 6.66 Dukes in energy, 3.33 in mass and time, and has just over half as much HP; a similar relationship exists with other factions' T3 Heavy artillery versus their game enders, which does almost the same job while being tougher and far less expensive. In other words, you could have built two or three heavy artillery which would have done the same job (albeit with less range, power, and rate of fire except seraphim) for far, far less cost in every way. Tier 3 Nukes are also very capable as they are extremely destructive, though they can be countered by enemy SMDs whereas Game-ending Artillery (and in most cases T3 arty as well) have no feasible defensive counter and the Yolona Oss is much less vulnerable to anti-nukes.
Really, the only reason you would build these is if there is a massive stalemate that even numerous nukes and T3 artillery can't break, which is highly unlikely in the first place.
Trivia
According to the optional in-game restriction, a game ender is classified as either a Tech 3 Strategic Missile Launcher or Artillery Installation. In other words, Experimental and other units (Strategic Missile Submarines) can still be used should a player choose to enable this restriction.