Got curious about Thaisha's switch up in her level 4 Feat, and man they changed Mage Slayer quite a bit in 5.5e.
Originally in 5e Mage Slayer gave:
When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.
When you damage a creature that is concentrating on a spell, that creature has disadvantage on the saving throw it makes to maintain its concentration.
You have advantage on saving throws against spells cast by creatures within 5 feet of you.
Now in 5.5e Mage Slayer gives:
Ability Score Increase. Increase your Strength or Dexterity score by 1, to a maximum of 20.
Concentration Breaker. When you damage a creature that is concentrating, it has Disadvantage on the saving throw it makes to maintain Concentration.
Guarded Mind. If you fail an Intelligence, a Wisdom, or a Charisma saving throw, you can cause yourself to succeed instead. Once you use this benefit, you can't use it again until you finish a Short or Long Rest.
So, some big shifts that also open it up more to those who aren't melee martial classes.
Reaction Attacks to Spells got scrapped, which isn't a big loss for Thaisha who doesn't rely on a melee weapon as her main method of attack. And getting that little bump to her Dexterity instead also raised Thaisha's AC. So that's really a trade up for Thaisha.
Disadvantage on Concentration Saves when you deal the damage has remained the same.
And then the big shift, really. Instead of Advantage on all Saves against spells only when they are cast within melee range, it is now an auto-success on 1 Saving Throw of Int, Wis, or Cha (not specified as coming from a spell) per Rest.
This last change really takes away from Mage Slayer being only (or near exclusively) useful for melee combatants who are hounding enemy mages. Mage Slayer will no longer help you with point-blank Chain Lightning or some shit, but it is useful now for keeping even your ranged fighters from getting whammied by enemy casters. And one good Save can make all the difference in a fight.


















