A game of two gimmicks: it takes place largely in total darkness, and the mechanics centre around wordplay. Among its many problems is that players may take quite a while to fully notice either of these.
Like earlier wordplay games (Nord and Bert Couldn't Make Head Or Tail Of It, Ad Verbum) Erebus mostly eschews coherent plot and builds itself around word puzzles. The tone is wacky Zorkian, and the writing's main strength lies in mildly amusing silliness. The setting is the main divergence from this: islands in a dark subterranean lake, with an atmosphere not unlike like the game's namesake Hunter, in Darkness. This, combined with the opening sequence, make Erebus feel rather like two games that aren't quite on speaking terms; a Zorkian wordplay game set in a wacky hell, and a moodily surreal game about darkness and silence.
The core gameplay is about constructing short words from letters that you've found. The most annoying thing about Erebus is that using a letter consumes it; a replacement appears where it was originally found, and this entails a great deal of unnecessary trudging around the map. The second most annoying thing is that some of its puzzle solutions, particularly towards the end, feel quite arbitrary; this exacerbates the first problem, because the only real way of working the solution out is to try making lots of different words. Erebus shoots itself in the foot by saddling its fun central mechanic with tedious makework.
In the hidden-object genre of casual games, there's a substantial and largely regrettable subgenre of romance-centred stories, within which are a few games about planning weddings. In certain respects, D-Day plays very much like Dream Day Wedding and its ilk. The gameplay mimics hidden-object in that there's effectively nothing to do except to find all the things and put them where they need to be. To-do lists and now-do-this instructions strive to preserve the player from the slightest confusion. The world is sparkly and conflict-free, and the bride is kept safely generic; her intended is absent, barely mentioned at all, and seems irrelevant to the whole undertaking. (It's probably unfair to consider D-Day as literature rather than a straightforward iteration of a standard fantasy; it's certainly not attempting anything more than the latter, which means that I'd be unlikely to like it much regardless of its other qualities.)
The two key elements of casual games that it lacks are a high level of aesthetic polish (in IF that might translate to lavish prose and meticulous scenery implementation) and an intuitive, idiot-proof, silky-smooth user interface. Both of these could have been substantially improved by testing.
It's probably fairest to think of D-Day as a My Apartment game. In that light, it's quite respectable: it demonstrates some thought about structure, it provides immediate objectives, it's trying to be considerate of the player, it can be won without ever getting stuck, it doesn't have the cynically lazy my-game-sucks attitude that typifies My Apartment.