To obtain a Basalt Pillar, you need to kill the Boss Geo Hypostasis on any World Level. Bosses are automatically Lvl 36 on World Level 0, so it is recommended to have your Characters at roughly the same level as the boss or higher.
To get the rewards for defeating this Boss, you must spend 40 Original Resin. Though if you have Condensed Resin stored, you can use this and get double the rewards.
To find the Geo Hypostasis, you can open up your Adventure Handbook, head to the Enemies Tab, locate Geo Hypostasis, then click on the Navigate button. The game will show you where the Boss is on your map.
Or you can head to the location shown below and cut out the middle man.
Just click on the image below to enlarge it.
- Type: Elemental Stone
Basalt Pillar is used for the following Character Ascensions.