Lets not number them this time. Posts are always assumed to be about the next one. Detailed homebrews will be superseded by the official version if one comes out.
1.Brain Welk. Aberration/Mollusk. This thing when implanted in the skull between the hemispheres attaches via nerve tentacles and usually takes over. While in control the creature has intelligence, wisdom, and charisma 20, darkvision, and however many levels of a class(often engineer) it has. Weather in control or not, the hoste body is purged of all other diseases by the single cell spores the welk creates.
Fresh from the dark tapestry, they are lawful evil, but someone who had one removed or just had a transfusion of spore filled blood, will have one grow usually with an appropriate alignment and personality. While many such become surgeons capable of implanting their fellow welks, some even become wizards.