Defeat all four Cretaphids and all four Slenches on Celestial Archieves, Alinos, Vesper Defense Outpost, and Arcterra.
You get an octolith each time you beat a boss however, If one of the hunters kill you they will take one of your acquired octoliths and you will have to get it back.
The Octoliths are needed to open the way to the final boss. All eight are needed in the Alimbic Cannon Control Room on Alinos.
You need to get all eight octoliths: they're the shiny diamonds you collect after each boss you defeat. The octoliths are the key to activating the cannon that will reveal the location of the planet where Gorea's hiding: Oubliette. When you activate the cannon, you will be transported directly to Oubliette to confront Gorea. Gorea comes in two phases: Phase 1 and Phase 2. Phase 1 is easy to get to. It's the phase that you have to beat first.
Once you have all eight octoliths go to the alimbic cannon room on alinos and then watch the cut scene as the oubliette reveals itself.
every time you beat a boss (8 bosses=8 octoliths=2 bosses per planet) they will drop an octolith. to get to a boss, you must find 3 of the 6 artifacts (3 of the same artifacts=1 boss portal opened) hidden on the planet used to power the boss portalthere are two types of boss (not including the final boss)slench : large and circular. hit the 3 "worms" that attach it to the wall about 3 times each with the correct gun (found out by scanning) and the slench will leave the wall, then shoot it in the eye to harm it. repeat until it is destroyedcretaphid : tall tower like and spinning. the cretaphid has numerous targets all over it's body which, depending on the version, will shoot slow green blasts that follow you and/or laser beams that rake up and down. the targets will be red (invulnerable) for a time while they are shooting but when they turn blue (vulnerable), that is the time to shoot them. when all the targets have been shot, the cretaphid creature (a small blue diamond that fires weak blasts) will eject out the top, and you will have a chance to shoot it, before it goes back in.repeat until it is destroyedafter you defeat a boss, it will show a cut scene of samus absorbing the octolith. upon exiting the room, a security protocol activates and gives you a time limit in which you must run back to your ship and take off before the time hits zero