Lectors are demonic beasts from another world. They have been summoned by Undead Archon to search for the Black Grimoire. They will often be found searching roughly through other books or hunting in places where the Black Grimoire has been or may have been, or possibly accosting people who may have some knowledge of the dark book.
In addition to claw and bite physical attacks, Lectors can attack at a short distance with a magical / electrical attack.
Uniquely among the beasts that Scarlett will face on her quest, Lectors can only be killed with her Moonblade. (Attacking them with other weapons appears to do damage, but they come right back to life after a few seconds.) Naturally, this makes them impossible for normal humans to deal with. (Despite that, Mental Skills that do damage, such as Blood Toll, work normally against Lectors.)
Lectors seem equally at home on two legs or crawling around on four of their six limbs.
Killing a Lector adds 80 points to Scarlett's Experience (each.)
Later in the game, Scarlett will encounter a more powerful variety of black Lectors. These are harder to kill (in medium difficulty, they have 800 health points to the normal lector's 280), but the XP gain is 80, too.