LOL well the ones on his arms he originally got because he heard having runes inscribed on your body can amplify your runic magic. Ro figured it wouldn’t be bad to invest in some. The particular ones on his arms are more so for manipulating blood and healing/regerating himself rather than draining his enemies.

The obvious one was gotten after he realized how bored he always was cause he couldn’t sleep around (though he did realize after getting the ones on his arms he could wake the beast by stroking it) He figured having runes directly on his dick would make it easier to get it up