n order to unlock this achievement you need to Decipher a total of 30 Eridian Slabs in the Campaign of Borderlands 3.
In order to decipher the Eridium Writings you need to obtain the Eridian Decoder which can be obtained by completing the Main Mission - The Great Vault.
Once you decipher them all head to the Sanctuary and interact with the Eridian Slab.
Video with all the Eridiun Slabs:
Timestamps:
Pandora
11 Eridian Writings
[0:00] The Droughts
[0:36] The Droughts
[1:01] Ancient Bluff
[1:29] Devil′s Razor
[1:55] Devil′s Razor
[2:24] The Splinterlands
[2:54] Carnivora
[3:25] Guts of the Carnivora
[4:02] Konrad′s Hold
[4:50] Cathedral of the Twin Gods
[5:23] Destroyer′s Rift
Promethea
6 Eridian Writings
[5:58] Meridian Outskirts
[6:37] Meridian Metroplex
[7:01] Lectra City
[8:17] Skywell-27
[8:46] Atlas HQ
[9:19] Neon Arterial
Athenas
1 Eridian Writing
[9:57] Athenas
Eden-6
8 Eridian Writings
[10:33] Floodmoor Basin
[11:06] Floodmoor Basin
[11:41] The Anvil
[12:04] Jakobs Estate
[12:27] Voracious Canopy
[12:57] Ambermire
[13:25] Ambermire
[13:54] Blackbarrel Cellars
Nekrotafeyo
4 Eridian Writings
[14:23] Desolation′s Edge
[14:57] Desolation′s Edge
[15:48] Tazendeer Ruins
[16:38] The Pyre of Stars