Points to Emphasize

This module very closely follows the 1e DMG as laid out on pages 115 – 116. I would direct you there to get an understanding of how it works.

There are a few things I would like to stress in regards to researching custom spells and the related module. I think they are important to maintain in your adjudication, though of course the decision is ultimately up to each DM.

Spell Level

It is up to the DM to decide what Level a proposed spell would warrant. It is recommended that you have your player(s) submit to you in writing the spell they wish to research in the same format as presented in the PHB. Duration, Area of Effect, Description, and so forth. Then you as the DM decide what Level the spell would be.

Overpowered

An Overpowered spell is just what it sounds like. A spell that is so powerful, it upsets game balance. That is solely for the DM to determine. If you deem a proposed spell as overpowered, do not inform your player. Simply do the calculations and then have the player invest the requisite time and money, with zero chance of success!

The DMG recommends you allow this research to go on indefinitely, until the player decides to call it quits. They will therefore never know for certain if their spell was impossible to create or they simply ran out of money or patience too soon. The AD&D Toolkit module notes the cost in time and money, should the spell have not been overpowered, in case you want to impose a finite penalty. It also offers the weekly cost, should you wish to follow the DMG and let research continue indefinitely.

Arcane Tomes and Scrolls

I personally make use of homebrew Arcane Tomes and Scrolls in my sessions. These are additional materials required, along with a library and lab. The DMG makes reference to “scrolls” and “texts” as being necessary Research Materials and it is this statement that my homebrew rules are based on. They are, of course, completely optional. You are welcome to include them in your sessions or not.