In the Aisaiger series, you play the role of an ordinary husband to a young(?) MILF-styled wife, where the latter gains the ability to transform when battling monsters and enemies keen on invading earth. Like all other eroge, the main protagonist will not be content with only one partner, and therefore, the producers see the need to include other MILF characters, albeit each with their own tragic history, into the monster-battling team.
When the notion of battle comes to mind, most people will assume Aisaiger to be a strategy-based game. How wrong can they be. One of the more interesting aspects of Aisaiger would be its card game feature. The player collects cards in addition to his/her default deck, which would then be used in battle against monsters, very much like the TCG commonly seen these days. Even losing has its benefits, especially for those with monster-fetishes, as our heroines would be placed in compromising positions.
Of course, Aisaiger is not all about monster-rape and tentacles. The bespectacled husband gets to have sex with his wife every night (at least in the first game) and during the day, he gets to travel to various locations, interact with other characters, which will eventually have an influence on the ending one may get.
In short: MILFs, monsters, human sex, card game... You name it, you have it... to a certain extent. :D
First game in the series - Aisaiger: http://www.getchu.com/soft.phtml?id=195101
Second game in the series - Aisaiger Powered: http://www.getchu.com/soft.phtml?id=195101
Third and last(?) game in the series - Aisaiger Flash: http://www.getchu.com/soft.phtml?id=195101
A thread on 4chan, which hopefully, still has some seeds for those keen to obtain the game: http://cgi.4chan.org/t/res/255508.html