Caster of Magic (wiki) is an expansion for Master of Magic written by individual developer Seravy (homepage). Originally a free-to-play Mod, Caster of Magic was picked up and re-published as a DLC by Slitherine Software shortly after their acquisition of the Master of Magic trademark from Atari in 2019.
Caster of Magic is a near-complete overhaul of the original game, rebalancing many of the mechanics in addition to introducing new ones. In contrast to the somewhat sandbox style of the vanilla game, Caster of Magic offers a more focused experience, with gameplay elements tuned to serve specific, clear-cut purposes. A number of less useful assets – units, spells, buildings, retorts, unit- and hero abilities, magical items and item powers – have been replaced with new, different ones, renamed, or redesigned entirely.
Diplomacy plays a much larger role in this expansion, as the AI is more advanced and relentless, providing a greater challenge – to the extent that being at war with multiple opponents at the same time can lead to losing the game very quickly. Difficulty levels, as well as enemy bonuses have also been re-scaled, and an overarching theme introduced, where the Wizard on the "other Plane" generally acts as a final opponent – although victory through casting the 
Spell of Mastery is still possible.
Caster of Magic contains all of the fixes and improvements of the Community Patch and more. The project is also under constant development, with players being able to discuss changes with- and report errors to the author on multiple venues – the most prominent being the expansion's forum on realmsbeyond.net. Caster of Magic is available for purchase from Steam and GoG, as well as the publisher Slitherine's own webstore, and requires the base game to play. Owners of the original hard copies are unfortunately not eligible however, and need to re-purchase Master of Magic online to gain access to the new content.
Because Caster of Magic has so many differences from Master of Magic, it has its own separate wiki – leveraging both the work already done in this wiki, as well as the Caster of Magic Manual –, which can be found at casterofmagic.fandom.com.