Indie first-person-terroriser Amnesia: The Dark Descent has raked in ten times the $360,000 it cost to produce, according to developer Frictional Games.

In a "21 months in" report posted on the studio's blog, Frictional Games co-founder Thomas Grip stated that optimistically, the game has sold 1.36 million units, including copies sold as part of the Valve Potato Sack and Humble Indie Bundle promotions. Excluding these offers, Amnesia: The Dark Descent sold 710,000 units on its own.

In terms of monthly sales, the game currently shifts 10,000 units at full price, even after nearly two years on the market.

'One would think that there is perhaps not much to be said this long after release, especially for a single player game with no built-in social features,' said Grip.

Grip attributes the persistent sales figures to the game's modding community, which have made the game the most popular game at ModDB as of writing, ahead of Half-Life 2 in second and Minecraft in third.

'Not only does this amount of user content lengthen the life of the game, it has also increased the amount of YouTube movies made with an Amnesia theme,' added Grip. 'It is quite clear that allowing users to create content is a feature worth putting time into.'

The developer also dismissed piracy as an issue faced by the company on the back of its strong sales figures.

Frictional Games' follow up title, Amnesia: A Machine For Pigs, is expected to be released in early 2013. It has been produced by Frictional and developed by Dear Esther creator The Chinese Room.

Frictional is also working on a 'super secret project' that will also be a first person horror title and will attempt to explore deeper themes than the Amnesia series.