Independent massively multiplayer game developer and publisher Turbine (Dungeons & Dragons Online, The Lord of the Rings Online) has collected $6.6 million in series D funding as part of a planned $50 million investment round.

The news, as usual, comes by way of an SEC filing. It was first spotted by various online finance-oriented sites, including peHUB.

Westwood, Massachusetts-based Turbine, one of the few independently-operated Western producers of major massively multiplayer games, has raised tens of millions of dollars in venture capital over the last several years. In 2008, it raised $40 million, much of it from GGV Capital and Time Warner. Other past financiers of the company include Highland Capital Partners, Tudor Ventures, Columbia Capital, and Granite Global Ventures, all of which are mentioned in the filing.

The studio has made headlines recently for its plan to turn its Dungeons & Dragons Online into a free-to-play game later this year.