RoM - Ars Goetia  

THE XML INJECTOR and Ability to Read

This mod allows you to use Natalia Auditore's Magic Circle to actually summon demons. For now, I'll include the object in this post, and I'll remove in case she updates her post with the customized version I made for the Mod. In case you already have the object, you need to replace it with mine (if you don't know where it is, or if you have it, just place mine in the overrides folder).

Every Sim can conjure a Demon by using the special rug (you can find it under rugs like a regular one, there are instruction on how to find it in the original post)

The options change if your Sim is a Spellcaster, a Vampire, or a regular Sim (there is something for mermaids too). 

There are 4 elemental Demons to call upon, and each one of them teaches a different school of magic (and has different options for vampires, too)

The Elemental of Earth (Pharyes Of The Mountain) teaches Practical Magic, the Elemental of Water (Undine Of The Lake) teaches potions recipes, the Elemental of Air (Sylph Of The Woods) teaches mischief magic, and the elemental of fire (Lamia Of The Flame) teaches untamed magic. They can unlock magic and teach also for vampire witches. 

All of them have the option to turn your regular sim into an occult type if asked for power:

Here is the fun stuff, though... you don't get to chose what they turn you into... It can be a Witch, a Vampire or... a Mermaid... different elementals have more chance to turn your sim into a specific occult, for example the Fae is more likely to turn Sims into Mages, Lamia (she's a Succubus technically) is more likely to turn your Sim into a Vampire (directly, without waiting, and with magic by default)... The Undine is the only one that has the chance to turn Sims into mermaids.

Vampires and Mages can ask for power, too... but they need to ask the Succubus or the Fae respectively... and they'll force you into a fight:

The XP gain is huge either that you win or lose this fight...

There is a cooldown of two days for asking demons gifts (they're way too powerful)

Be ware that the summoning can fail...