Aefein Dubdam Aren (Tome and arcane focus)

Sometimes also known als Zaumuth's Ruin. Once belonge to the Dubdam Aren (or fallen Queen), known in her time as Laelyrr Zaumuth. The Fallen Queen was the last great Drow queen who made alliances uniting A large part of the underdark and Marching on the overworld. Secretly, she had made a Pact the great old one and the march on the overworld was a coverup to release him. Laelyrr was however betrayed as some advisors and generals plotted to stop her madness, they slew her and the Tome has been put deep in the heart of the castle at the centre of a magical labyrinth where Zaumuth's kin are tasked to gaurd it forever as pennance for Laelyrr"s acts.

When attuned, it allows you to Plane shift into  the great old one's demiplane using your Highest level spell slot under 7th once a day, (so 6th level or highest you can cast) the Great old one can push you back out at any point after 1 minute if it so wishes

Contains the secret rite by which to release Great old one from it's binds (locked in a demiplane book is gate) and as well as the other artifacts needed for it. Reading takes a lot of time (at least 80 hrs) and Gameplay bonusses can be unlocked through knoledge withing (sort of Tome of Evil but with Eldritch flavor)

Can be used as an arcane focus for any spells (for example by wearing it on your heart and summoning the spell effects from it)

Notes: in order to attune a pact has to be made with the Great old one (to be named) who resides within, Cursed, Sentient, Focus