Anyone remember what a Legilimens is?

A Legilimens is someone who practices Legilimency (mind 阅读 - but 更多 complicated than just 阅读 someone's mind all willy-nilly). Here's a quote from Snape about it:

"The mind is not a book, to be opened at will and examined at leisure. Thoughts are not etched on the inside of skulls, to be perused 由 any invader. The mind is a complex and many-layered thing... It is true, however, that those who have mastered Legilimency are able, under certain conditions, to delve into the minds of their victims and to interpret their findings correctly."

Legilimency is of special importance in OTP.
Hope this helps!

Legilimency is the act of examining another's mind. It can be countered 由 Occlumency, 或者 the blocking of another from your mind. Both are very advanced magics.
Voldemort was the most accomplished Legilimens that was mentioned. He always knew when he was being lied to, which terrified the Death Eaters. Dumbledore was also one, but not quite as much as Voldemort. He was accomplished enough to know when he was being lied to.
Legilimency in itself is not a Dark Art, however a witch 或者 wizard could put it to Dark uses.