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You mean the Black Guardians of Ulthwé, and the rule is exactly where GW left it-- in the EoT campaign book. It was never in an official Eldar codex, it was exclusively for the EoT Ulthwé Strike Force list.
It's GW's copyrighted intellectual property, so it can't be posted in detail. In general though, they had a Webway Portal very similar to the Dark Eldar, they had a unit consisting of an Avatar and a Warlock bodyguard, they had a special warlock power, and their guardians had higher WS and BS. They also had restrictions on the number of Aspect Warriors they could take. If you're interested in looking it up, the only thing I can recommend would be trying to find a second-hand copy to check it out for yourself.
"Codex: Eye of Terror". There are a couple of them up for sale on Ebay at this moment. However, be aware that the EoT Codex is no longer legal to use except in friendly games, and may cause some difficulties with compatibilities for 5th Ed rules.
"Tau Commandment #226: Participants who use Velocity Trackers in the Tau Clay Pigeon Tournament will be disqualified"