I feel like entropy (see also: chaos) as a magic/element/damage/whatever type isn't that unusual, and it's usually partnered with order/creation (or some synonym for one of the two) if it's in a system that has opposing pairs. Well, the use of the name is not uncommon, at least.
Actually using it properly thematically is another story entirely, and as has already been pointed out, it's totally not being done here, either; it's called "entropy" entirely because it sounds cool. The spells in its category so far generally have less effect on overall entropy one way or the other than most other spells do, amusingly enough. And "what he/they said" about light as its opposite.
On the other hand, it's no worse than pretty much any other fantasy/magic game out there, and I never would've said anything about it if no one else had. Heh.