You learned the ways of summoning other-planar entities through sacrificing living beings to them.
With a 1-hour long ritual, you can sacrifice one or multiple living creatures with souls. When you do so, you can do one of the followings things:
Conjure Entity. Choose either the Fiend or Aberration creature type. You conjure one or multiple creatures of that type with Challenge Rating of 10 or lower to your plane. To do so, the combined Challenge Rating of the summoned creatures must be equal or lower than the combined Challenge Rating of the sacrificed creatures. The summoned creatures are friendly to you and your companions for 1d4 hours and obey your commands for that duration. If you give no commands, they defends themselves but otherwise take no actions. At the end of the duration, the creatures act in accordance with their normal disposition and alignment.
Otherworldly Knowledge. You cast the contact other plane spell without requiring to roll the Intelligence saving throw against becoming insane and the legend lore spell without requiring the material components.
Supernatural Expertise. You gain proficiency in a skill of your choice for 24 hours. Your proficiency bonus is doubled for any ability check you make that uses the chosen skill. Once you used this ability, you can’t used it again until you finish a long rest.
