In Colossal Arena there is a group of eight creatures that are fighting. Each round, one of the creatures will die. To decide which unlucky soul will be the victim, players put numbered power cards in front of the creatures, with the lowest one going the way of the dodo. But what makes this game interesting at all is that players can make five bets throughout the game which will sometimes allow them to use a creature's special power and gives the game a method of scoring at the end.
Titan: The Arena is actually a reworking of a Reiner Knizia game called Grand National Derby, but Avalon Hill's remake was quite significant from a gameplay standpoint.
The Titan: The Arena printing is often confused with its namesake, Titan. But other than the fact that they both use fantasy creatures as a general theme, there is very little that is similar between the two.