If a mage casts Feeblemind on a warlock with Thought Shield, does the mage take any effects on the spell besides the damage? Does any of the Int/Chr drain reflect back on the caster?
Feeblemind (PHB p.239)
You blast the mind of a creature that you can see within range, attempting to shatter its intellect and personality. The target takes 4d6 psychic damage and must make an Intelligence saving throw. On a failed save, the creature’s Intelligence and Charisma scores become 1.
Thoughtshield (PHB p.110)
Starting at 10th level, your thoughts can’t be read by telepathy or other means unless you allow it. You also have resistance to psychic damage, and whenever a creature deals psychic damage to you, that creature takes the same amount of damage that you do.
Does Thought Shield affect Feeblemind?
Feeblemind does 4d6 psychic damage (no save) and then requires the target make an Int save or their Int and Cha become 1. So YES it does, Thoughtshield grants resistance to the psychic damage and the caster takes the same amount of damage the target does (i.e. half of the 4d6 roll). However it has no influence whatsoever on the save or the effect on the target's Int and Cha should the save be failed.
The Thought Shield only states that it reflects Psychic damage and prevents others from reading your mind. It says nothing about other mental effects (which aren't explicitly called out, either) so it will not reflect any other type of damage or effects.