The 'name presenting' ceremony they mentioned.
In another series whose name I won't mention cuz it's a spoiler, it's a ritual where a person would carve his 'name' into a magic stone, fill it with his magic power, and present it to the one he recognizes as his master.
The master will then envelop that stone with their own magic power, forming a sort of magical bond between them.
In essence, the master can order the other side to do whatever they want, and the other can't disobey at all.
As a side bonus, the subordinate is also affected, slightly, by the master's magic power, which can indirectly give them a small power boost (or increases their elemental affinity since the masters tend to have more than them)
Although in this case, it might be a ritual to learn the Demon King's true name, and in doing so allowing whoever did this to call him sorta like a familiar.
Cuz that's the thing that's been getting censored all these times, and he's been mentioning waiting for her to call his name.
Then again, in that other series there's also a different ceremony to learn the Supreme Gods' name (which apparently is different to each individual, and whoever reveal the names they learned will be forever shunned by said Gods)