In the official Champions Universe, Taipan is a superpowered assassin for hire, considered by many the deadliest assassin in the world. From 1997 to 2000 he's known to have killed nearly two dozen people, more than half of them superhumans. Super targets was a specialty of his; VIPER, ARGENT, and the Warlord all employed him on multiple occasions.
In 2000 he was captured by the Chicago superhero team, the Peacekeepers, after being hired to assassinate all of them (he did succeed in killing two). He was tried in Australia, scene of his first known murders, and sentenced to a lifetime in Hot Sleep in "House Taipan," a super-prison specially built for him in the Outback. He's been there ever since.
IMHO this guy should have been broken out by now and be terrorizing CO PCs. Since he's a mercenary he could show up in the employ of almost any villain or villainous group. He can be part of a story arc involving other villains, or be featured in an Alert. His true identity and background are completely unknown, so can be developed as needed for story purposes.
After Champions Online broke a whole slew of villains out of Stronghold, springing one more doesn't seem a lot to ask.
(Taipan is fully written up in Champions Villains Vol. 3: Solo Villains.)