“One Ring to rule them all, One Ring to find them, One Ring to bring them all and in the darkness bind them.”
Crafted in secret by the Dark Lord Sauron in the fire of Orodruin, the ring contains a fragment of his power. Contained within is the power of a Maiar; those who wear the ring feel its power seeping into them, gaining the might once held by Sauron.
The most basic and first power to manifest is invisibility, except for those who can see beyond the non-physical mortal plain. This invisibility is not true invisibility, only shunting your body between dimensions so as not to be seen.
Following the manifestation of the previous power, you would see a noticeable increase in defense when wearing the one ring, gaining an enhanced body being able to bear attacks far greater than previously able to, as well as enhanced senses and immortality given so long as the ring is worn or near the bearer.
The previous two abilities would be projecting a false vision of reality to others, appearing superior to before, affecting the minds of others, and granting inspiration to others. Furthermore, you also gain the ability to naturally understand the speech of evil creatures and an increased understanding of language in general.
Should you master the power within the ring, the ring will acknowledge you as its new master, allowing you access to the power hidden inside the ring augmenting your abilities to further heights, such as the power of the Maiar. Sauron, held inside, can Dominate others, in addition to his other abilities as a Maiar. This ring is loyal now only to you so long as you have gained mastery, and the sentience of the ring corrupts all others wishing to get back to you. The ring has all the powers of the other rings, but to a far greater extent given that it was created with the might of a Maiar.