Forces outside of your control are attracted to you like a moth to a flame. Biological creature would see you as their perfect body, cosmic forces would run after you like as if you were their soulmate, magical artifacts would feel as if you were their destined user, and if you somehow injected yourself with power-granting serums that have a 1% of chance of success, they would fit you like a custom-made clothing; even if they had side-effects, you would be immune to them. Strong enough forces can resist you and even retaliate if you go after them, but eventually, your powers could grow so much, that even beings like the marvel Phoenix Force would look at you as if you were the second coming of Jean Grey, even if you don't have psychic powers.
You start being able to only have at most 3 entity existing into your being, have a pocket dimension where you can store those you are currently unable to use and you must abide by the contract of every being that decides to be with you, but you also can reject to have them, unless they require nothing. As time goes on, you will eventually be able to host more and more entities at the same time, until you can have an unlimited number of them at the same time.