The most obvious would the Stormbringer! roleplaying game in its various incarnations. My favorite supplements for this series would be the Atlas of the Young Kingdoms v. 1, The Unknown East, and Sorcerers of Pan Tang. To that you can add the MRQ books Elric of Melnibone line (by Lawrence Whitaker, primarily) are more accurate to the world in the Moorcock stories. These include the titular book, the Companion, Cities of the Young Kingdoms: The South, Cults of the Old Kingdoms, Dream Realms, and Magic of the Young Kingdoms. The MRQ books in particular have the most developed version of the setting and systems, and are easy to adapt to Legend, OpenQuest or Mythras - with some modification - being somewhat closer to them on the BRP tree than classic Stormbringer!
Lankhmar Unleashed for MRQ has some interesting features, though given its reliance on MRQ1 GMs will probably want to abandon many of the internal mechanics for ones native to the system they're using.
BRP Magic World is very much like Stormbringer without the serial numbers, but benefits from further editing and refinement. The default setting may not be as compelling to some as the world of Elric, but can easily be substituted with the Young Kingdoms, Hyborean Earth, Xothique, etc. with some minor modification. If nothing else is magic, setting and character abilities can serve as inspiration or be adapted to anther BRP system quite easily.
BRP's Mythic Iceland is more mythology, but it happens to be a type of mythology that was very influential on writers like Poul Anderson and thus has a lot of resonance with traditional S&S settings. In a Hyborian Magic World game, for example, Mythic Iceland could serve as a resource for the Vanir and Aesir lands.
Vikings of Legend can serve a similar role. While Mythic Iceland is close to core BRP products, Vikings of Legend is just as close to MRQ2/RuneQuest6/Mythras, and may be easier to use as a resource for someone working with those systems.
Crusaders of the Amber Coast is a largely historical medieval period game involving Baltic crusaders and their environment. Its atmosphere of sinister pagans in the wild bogs and forests (think Conan's Picts) and relatively brutal military conquest in the name of a divinity could be easily adapted to dark fantasy. It is before the high medieval period, making it closer to the ancient/medieval hybrid of most sword & sorcery.
Call of Cthulhu is the most distant from any of these systems, especially in its 7th edition. However its magical systems are semi-compatible with most BRP games, and the pre/posthuman god-monsters and black sorcery of Lovecraft are very much appropriate to most S&S games. The same can be said for Raiders of R'lyeh, which has rules more appropriate to pulp characters than CoC's glass-boned scholastic types.
Clockwork & Chivalry and its later version Renaissance Deluxe are much more technologically advanced than most sword & sorcery (though there's no intrinsic reason you couldn't have dark fantasy in the high Renaissance) it does have some unusual, medieval-themed spells that may be more suitable for a S&S game than fireballs and djinn summoning.
Finally, the Thieves' World Sanctuary Boxed Set - if you can find it - has a lot of good information on a dark fantasy urban campaign and in particular the setting's city of Sanctuary. The character statistics are converted into many different systems - including RuneQuest - which can be useful in another respect: this can help the GM to look at other dark fantasy games such as Astonishing Swordsmen & Sorcerers of Hyperboria and bring those characters, spells and abilities into a d100 system by comparison.
Surprisingly there has been no direct adaptation of Conan to any d100 system, nor of any impersonator like Thongor. There is a GURPS 3rd edition solo adventure set focusing on Conan and his world, but as complex as GURPS and d100 games can be the conversion work would be more than it would be worth.
My ApproachIncidentally, the way I would do it? I would use Mythras as the core, turning Animism into something more like Summoning with Manifestation traits; use MRQ Elric for a setting; supplemented by creatures, adventures and spells from Stormbringer (as needed); make Lakhmar and Sanctuary cities in the Young Kingdoms; make a Hyborian viking race from the Far North based on Vikings and Legend, again with spells and creatures drawn from Mythic Iceland as needed..
This would require some moderate conversion work. In magic I would favor going with Mythras magic/spells/powers when a version exists or can be adapted with existing effects. Animism, Sorcery and elements of Mysticism would make it more appropriate - note this would make the magic system more like Magic World than the various Stormbringer and Elric games. Otherwise, use the MRQ version adapted to Mythras. Only convert Stormbringer! stuff if it's especially neat or fitting to the feel of the game.
Finally, monsters from Raiders of R'lyeh and Call of Cthulhu would be especially appropriate adaptations if characters are sorcerers, demonologists or witch hunters.
Using the Elric/Stormbringer supplements as the core is easiest since so much of it is close to Mythras, and so much of it exists in a BRP format. While it might not get the exact 'power levels' correct doing quick, rough conversions where percentages are mapped on, characteristics are copied over, and rough ability in spellcasting are plugged into the conversion is an easy way to loot the vast hoards of BRP-type materials, much a D&D-style games are famous for. The exact distinctions in the game systems characters are built with and for can be ignored, though the result will be novel compared to the original iteration that is often more interesting than a rigorous conversion would be.