Eldtritch Sorcery
Hi folks, thanks for the interest in ES.
The goal for ES was to create a well-mixed set of spells for use in a wide variety of campaigns. As the developer and lead author on this book I certainly don’t make the claim that ALL of the spells in the book should fit into your campaign. That is a decision for a DM to make. For example, Wizard spells are not balanced by school so a specialist wizard (depending on the specialty) could get a serious power-up using spells from the book without prior DM approval. I rather think that is the case with the Spell Compendium as well though

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In the book I do mention things to watch out for. There is no way to account for the vast number of prestige classes, special abilities, and feats available in other products. If you (as the DM) allow things to stack without watching them carefully players could probably abuse things.
All of these things said, I believe the Spell Compendium and ES are certainly compatible and both can be used together. Personally, I believe that any spells from a source outside the PHB should have DM approval. It just makes things a bit easier in the long run.
There are spells in both books that can, in certain circumstances, be abused. I believe that the spells in ES are set at the appropriate levels. A few might be a bit powerful, but in actual play some of the spells I have seen complaints about simply haven’t been that bad.
Spells in ES range from pure damage spells to spells designed more for use in role-play heavy campaigns.
Special Note: We do have a Tome of Artifacts scheduled to come out in November/December ( I believe). This is a book developed by Ari Marmell that includes heavily detailed artifacts as well as ways to introduce them into a campaign. Many of the artifacts are very interesting, in other words, not all of them are simply powerful weapons.
Patrick
PS. It is difficult to write with a 3-year-old singing in the background. If something didn't make sense I'll try to check the thread again later.