The Abyss (aka Reshanta) is a public zone available to both factions. Once you reach 25 you will need to do the Access Quest (no that hard). After that you can enter the Abyss from any of the three Abyss Gates your side controls. They are at the Fortresses.
Yes you can go there Solo, but once you step outside your side's fortress you are a snack if you run into a gank squad or anyone higher than you. it is a War out there.