The ring of fire is just visual so it adds nothing to the skill, its just there to kinda let you know the radius of it. Sometimes you get hit by a big hit so you go into your hit recovery animation, while you were casting the colosseum, so it interrupts the skill and even though it shows the skill effect, it does not actually go through. This happens with pretty much every skill in the game, not unique to colosseum.
When you colosseum into a pack of mobs, it makes them have -25% physical resistance and it lowers their movement speed. It can also be used as a movement skill to pass from pack to pack with enough point investment.