An altogether more comprehensive RPG than T & T or others. This RPG was published in four volumes over easily as many years and has long been out of print. It tries to be more realistic (if one can have realistic fantasy) with more than a nod to 'proper' wargaming. There are signs of interest again in this RPG as people rediscover it and realise that its adult approach chimes with the more mature fantasy now being written, e.g. Game of Thrones and the Malazan series.
Bifrost is a complex RPG system dating from the late 70s and has a distinctly British (or even more parochially, English) feel to it. Whilst it is a serious game there's an understated humour that underlines it.
The authors of the original three volumes - Faerie, Combat and Magic were G.Highley, S. Johnson, K. Minear and K.White and little more is known about them.
After many years of being moribund there seems to be a few signs of life with a number of devotees of old forgotten RPGs (and Bifrost certainly falls into those categories) seeking the books.