The Forgotten Tower in Mortal Shell 2, a location sequestered in the southeastern reaches of Mammon, holds a significant secret: the Old Prisoner, a colossal bird-like entity appearing deceased. Scattered messages within the tower hint at its importance and suggest returning in Night Mode. A map found here highlights three distinct locations where essential Glands can be found.
Locating the Forgotten Tower
The Forgotten Tower is accessible via Bone Gate teleporters found in Sester's Retreat and The Silent Steps. The Sester's Refuge offers a more convenient route, with the Sester's Gate Beacon being the closest. From this beacon, navigate to the refuge's courtyard, descend to the rocky ledges below, and utilize the Bone Gate teleporter to reach the tower. The tower itself features a central structure housing the bird giant's corpse and a bridge leading to a smaller building. Besides cryptic messages, this smaller structure is where the Axatana, considered one of the game's best weapons, can be found guarded by a Gloom Seeker.
Acquiring the Glands
Three distinct Glands must be collected: the Thawed Gland, the Depleted Gland, and the Faded Gland.
The Thawed Gland is located within the Conquered Temple, the dungeon culminating in the boss fight against Droeg, the Conquerer. Upon entering the dungeon, proceed until you reach an indoor area atop a ledge overlooking a snowy bridge. Inside this area, amidst stone hands and a Wraith, the Thawed Gland rests on a candle-lit altar.
The Depleted Gland is found in the Withered Shoals dungeon, just before the Hexapod boss arena. Progress to the ledge where Ruk is situated, then drop down one level. Instead of entering the boss arena, turn around and walk towards a cave where the Depleted Gland is plainly visible.
The Faded Gland is situated near the Citadel Annex Beacon, within the game's final dungeon, preceding the battle with Solnir, the Gloom Seeker. Leap from the ledge bordering the Beacon to the ground below, using the Harden ability mid-air to mitigate fall damage. After dispatching the surrounding Gloom Seekers, the Faded Gland can be found on a small altar.
Activating Night Mode and Offering the Glands
Once all three Glands are collected, Night Mode must be activated. If you do not possess the Gloombound Flame, found near the Great Arbiter of Flesh, it must be acquired first. Return to Marrow Keep and ascend to the tower's summit. Use the Gloombound Flame to ignite the brazier, summoning Thestus, an NPC from the first Mortal Shell game. 
Approach the bird giant, and in Night Mode, you will be able to present the three collected Glands as offerings. This act rewards you with the Seedbearer's Scripture.
Upon receiving the scripture, the bird prisoner will vanish, concluding this questline. The Forgotten Tower itself is a callback to Mortal Shell 1's Fallgrim Tower, which served as the game's central hub. In the original, the Old Prisoner was the final boss, attempting to achieve godhood with the Glands, only to be defeated by the Foundling.
The Seedbearer's Scripture
The Seedbearer's Scripture is an equippable item designed to counteract Cosmic Disease, a status effect primarily inflicted by the game's final boss, Zmey, the Unbidden.
When Zmey unleashes his fiery breath, he applies Cosmic Disease, which can eventually incapacitate your Harbinger, preventing the use of Shells, skills, and Tarstones. Equipping the Seedbearer's Scripture grants immunity to this effect and releases a wave of energy that temporarily stuns Zmey. Currently, only one Seedbearer's Scripture is available in Mortal Shell 2.