In D&D, Foulspawn is a race of humanoid aberrations that emerged from the Nentir Vale setting and its World Axis cosmology. They are the tortured remnants of human beings who were innundated with the unwholesome energies of the Far Realm, warping them into monstrosities. So thorough is this corruption that Foulspawn become absolutely disgusted by the world which they once belonged to; any plane that isn't part of the Far Realm strikes them as being inherently wrong, non-aberrant life is a heresy that must stamped out wherever it exists. To this end, they flock to each other, and to the service of mightier aberrations like aboleths, beholders and illithids, seeking to destroy and corrupt all that is not like themselves.
Foulspawn were expanded upon in an article technically released as part of Dungeon Magazine #194, but which can still be found online here: https://www.wizards.com/dnd/files/194_MMUpdate_Foulspawn.pdf
The "Starspawn" of 5th edition's Mordenkainen's Tome of Foes, despite their names, are actually a cross between an update of the Foulspawn of 4e and a catch-all for more overtly Lovecraftian aberrations; hence the Larva Mage being rebranded as a "Starspawn".
In 40K, Foulspawn was one of the most diseased and disgusting of all Nurgle's Champions. It is said that he was so horrifying to behold and approach that those who tried to fight him were incapacitated by nausea and gagged and wretched in disgust. Nurgle was deeply fond of this disgusting champion and wound up gifting him with a change into That-Which-Must-Never-Be-Named. As is known by now, most creatures turned into unspeakable beasts perish within a few hours, as their biology is now that of a creature that should not be, or, more commonly, they're BLAM'D by their former comrades.
Foulspawn was an exception to this rule. Not only did he survive, he grew stronger and more powerful. By either favor of Nurgle or random whim of chance, Foulspawn's new, toad-like form was able to survive by consuming living creatures, an action Foulspawn accomplished usually by swallowing them whole. In time, Foulspawn grew not only in strength and size, but in intellect, and to this day remains the only known case of an unspeakable beast that was then ascended to daemonhood by the whim of father Nurgle. However, his proclivity towards devouring both friend and foe alike did not endear him to Mortarion. As much as he loathed Foulspawn for carelessly eating his sons, the Death Lord could not destroy him without offending Nurgle. Instead, he banished Foulspawn to the wilds of the Plague Planet, where it has since grown to the size of a mountain.
His real-life counterpart would be Luke.