These glass pens are most commonly made from Amniteum glass by skilled Naldorese craftspeople. They have a reservoir that is always fully filled with a black ink that flows through spiralling channels into the tip of the pen itself. The reservoir never seems to become empty no matter how much words have been written with the pen.
Ink from the Sea.
Upon close inspection one might see the glass reservoir itself seems to be churning slowly as if it were a miniscule maelstrom. This is caused by an enchantment that creates a tiny arcane portal through which the ink enters the reservoir. The enchantment for the other side of this portal is put on a projectile which has to be shot into the ink sac of a kraken. The ink is tapped from this kraken into the pen as long as the kraken is alive, and the enchantment and the pen itself are not broken.
Craftspeople and arcanists have tried to create a similair effect with enchantments connected to barrels of ink or smaller cephalopods that produce ink, but have had no success in creating functional pens. Scholars have theorised that the ink sac of a kraken may have some innate power which is needed for the enchantment to work. As one might imagine, implanting the projectile into a kraken and living to tell the tale is a most dangerous undertaking. The craftspeople do not do this themselves but hire adventurers and hunters to do it for them. The combination of being made of such a fine material, the magical enchantments, and the danger for loss of life involved in making these pens functional, makes them extremely valuable.