Mayhem is a vintage electronic marker ranging from the turn of the century that just never menaged to catch on. Reasons aside, the last version was made in 2003 (pictured on this site) but it seems to have only been available for a few months after release. I recall attending an event in 2004 where I noticed a few unrelated vendors selling clearance Mayhems that they had purchased dirt cheap, for around $100 each. These days Mayhems are collectors items due to the rarity, and their value has risen to the range of $200-300 depending on condition (at the time of this writing, anyway).
The design of this marker is similar to its parent Nova, except this marker uses a blowforward closed bolt operation instead of the Nova's moving breech. Essentially the firing mechanism is the same between the two guns, except heavily rearranged and modified.