Target 1 Special Summoned monster your opponent controls: Equip this card to that monster; it cannot change its battle position or activate its effects. When this card you control is destroyed by a card effect and sent to the Graveyard: Destroy 1 Special Summoned monster your opponent controls. You can only use 1 effect of "Death Trap Mutation" per turn, and only once that turn.
So basically a Fiendish Chain that also snipes the opponent's monsters if they're foolish enough to MST it.
Given how SS-oriented the game has become, I can certainly see this being used quite often. More often than not, unless you're against Monarchs or something, there's going to be at least one SS monster on the field for you to target/destroy.
Unlike Chain, opponent can still attack with the affected monster, but suppose you can use it to keep monsters in Attack Position so you can hit them for damage, or something along those lines.
One thing I could probably recommend is making the destroy effect optional; most of the time, you probably will want to pop something, but more/less to give you some flexibility in its usage.
(Card description at top says Cont. Trap, but this should be a Normal Trap; at present, all Trap Cards in the actual game that equip to monsters are indeed this)