Pleasure runes are magical sigils which are used by monster girls in order to amplify the pleasure they feel during sex. They're almost exclusively seen as tattoos on the skin, although recent developments in Sabbath magic also allow them to be embroidered in items of clothing. Most depictions show pleasure runes as being in black ink, although white and pink varieties crop up occasionally too.
Since monster girls are already extremely sensitive to pleasure by virtue of their succubus bodies, the decision to get inked on top of that seems indicative of hedonistic depravity above and beyond even the usual (very very high) level. Daily life becomes difficult when even a pat to the head leaves one a sopping wet mess of arousal and rape-lust. Dark Angels are singled out as regular users, unconcerned by their reduced ability to function with anything like normalcy.
The war-tattoos that cover Amazonesses' bodies are actually non-functional pleasure runes, incorrectly copied down from the succubi that first raped the tribe into submission. Fairy Powder is specifically cited as one of the ingredients used in pleasure rune ink. While not directly referenced as a pleasure rune ingredient, kraken's ink is said to be a precursor for various magical recipes, so it might also be used.
These are the magical seals that the succubus (p.8) and dark priest (p. 206) featured in this book engrave into their bodies. These exponentially amplify the pleasure received by the target with the runes engraved on her flesh, so they're mainly used by confident and highly capable monsters to enhance personal enjoyment of sex. A distinctive feature is that the increase in pleasure corresponds with the size of the area of the monster's body that is engraved with runes. The size used by the succubi is typical, and allows them to function in daily life and have sex normally, while enabling them to experience even greater pleasure. Those which cover half their body in runes, like the dark angel, do so because they have all but entirely abandoned daily life, being constantly engaged in sex. Monsters with that many runes experience sexual pleasure just from being patted on the head by a man, and stimulation of their ordinary erogenous zones fills their heads with a dizzying ecstasy. There probably isn't anyone willing to try it, but if one were to cover her entire body in runes, the pleasure would be so overwhelming that she'd almost climax just from a simple touch, to the extent that everything else but pleasure would be erased from her mind in that moment. Essentially, since demonic energy is needed to use them, they can only be used by monsters, and they're only engraved into the bodies of monsters. A human woman wouldn't be able to stand the pleasure induced by the runes, rather, the power and pleasure of a rune would cause her to transform into a crazed monster who thinks only of pleasure in the middle of the engraving process.
“We've made a recent advancement that allows us to also embroider runes into clothing and still retain the full effects. Those with a childish, sensitive body like me don't have a lot of excessive extra flesh, unlike some others. Even without it, we can still have lots of fun with elder brother. It's something I have no use for. What? ‘Say that after finding a partner?’ Bite me!”
Furthermore, as for their body paint that also looks like illustrations, they claim “it's a charm to draw out the power of a warrior.” But actually, when the amazoness tribe changed from human to monster, they were taught of “pleasure runes” by the succubi who invaded their villages. After being inaccurately passed down, the runes ended up becoming altered.
Originally, the purpose was to amplify the pleasure of sex for the person with the runes engraved, so it was made so that the succubi could further enjoy sex, and had absolutely nothing to do with battle. If it had accurately been handed down, then the amazoness would now have pleasure runes engraved all over their body. With their entire body attacked by intense pleasure, they wouldn't even be able to do daily life activities, much less fight. Instead, they would be unable to think of anything but sex, and they'd end up shutting themselves in their homes constantly having sex with their husbands.