Avenger is a game based on Defender, but with restrictions and additions of it's own. Unlike Defender, you can not reverse your direction, and there are no humans to rescue. On the other hand, you also get waves where you have to battle HUGE motherships, that as soon as you hit them, they become a swarm of smaller aliens (the equivalent to mutants). It also pretty well has the closest "feel", as far as gameplay and action goes, to the original Defender. It also has fairly decent sound effects.
The Cornsoft Group, the company that published it, was originally quite famous for various games that they pubished for the original TRS-80 Model I/3/4's including such hits as Scarfman. They ended up doing several ports of their older TRS-80 games to the Coco, as well as some originals, like Avenger.
Title: Avenger
Author: ???
Publisher: The Cornsoft Group
Released: October 1982
Requires: Color Computer 1,2,3, 16K RAM, tape or disk.