This pack was made for The Abyss: Chapter II mod, version 1.9.7. It will probably work for later versions unless they change where the monolith texture is located or what it's named. You can check out the mod here https://www.curseforge.com/minecraft/mc-mods/the-abyss-chapter-ii
This pack aims to make searching for Letter Spawn Monoliths much easier by making them a bright red color, which is very easy to spot amongst the black and cyan color scheme of the abyss. Additionally, it removes the overlay when in ghost vision so you can fly around with the ghost ring and not miss a single monolith. Since there are little to no red colors in the Abyss, with the closest being node crystals which are pink, it will be hard for you not to find these monoliths, and you will soon realize that they are not necessarily rare, just hard to spot with the default textures.
Letter Spawn Monoliths are very important in the abyss as they give you lost letters, which are essential to summoning the final boss.