I had never heard of a spell called Backlash. I found out it is from Heroes of Faerun but since I don't own a copy, if someone can post spell's description? It would be helpful.
Level 4 Abjuration spell that gives 4d6 damage reduction as a reaction to taking damage. You can ping that damage back at the creature that inflicted it on a failed save (half as much on a success) if they're in range.
4d6+ spell mod damage reduction, so about 17-19 damage reduction based on your spell mod.
4d6 damage = about 14 damage, 7 on save, average of about 10.5
I don't think anyone is arguing it doesn't do things. Yes, it reduces damage and reduced damage can have you avoid making a concentration saving throw, it can keep temp hit points up longer etc. but it is so situational it is probably not worth one of your very limited known spells.
And sure there is some synergy with armor of agathys, but that is a lot of your limited slots going to one backlash effect. Until level 11 that is literally all your slots just to keep armor of agathys going for oneish more hit. In almost every case you'd be better off using your other slot with a different spell once you already have agathys going. Sure eventually you have more slots to spare or maybe this is right after a magical cunning use so agathys is used and you now have 2 slots. but this is just getting so situational I don't think its worth one of your known slots. But its a game its about having fun so if you think that double whammy would be super fun, have at it.
Casters at level 11+ are usually concentrating on a powerful spell, and monsters are dealing 28-55 damage at that level. or even lower. A cr8 Frost giant is doing two attacks, likely with advantage, and a +9 bonus. this means, even with a 15 AC (14 dex and mage armor)and shield spell, they have about a 70% chance to hit you. Not to mention breath dex saves, etc.
its not as much of a unlikely situation as you present, So this spell maintains the level 4 or 5 spell you cast, which often requires an action or BA to cast, as well as dealing damage and reducing damage taken.
Thats without even considering agathys, or similar things. Like straight up logically speaking this spell would always be a better choice if its preventing a spell of 4 or higher from being broken that costs an action or BA, because itss maintaining the spell, reducing damage taken, and dealing damage, whereas recasting that spell without doing it costs more time, and you take the damage, you deal no damage, and has the same spell cost.
Especially if you are a mid/close range mage, this is a valuable spell, as you are bound to take hits eventually.
And remember, every spell is not for everyone in every usecase, the vast majority of spells you select are situational.
4d6+ spell mod damage reduction, so about 17-19 damage reduction based on your spell mod.
4d6 damage = about 14 damage, 7 on save, average of about 10.5
can be upcast for 1 d6 per level.
Casters at level 11+ are usually concentrating on a powerful spell, and monsters are dealing 28-55 damage at that level. or even lower. A cr8 Frost giant is doing two attacks, likely with advantage, and a +9 bonus. this means, even with a 15 AC (14 dex and mage armor)and shield spell, they have about a 70% chance to hit you. Not to mention breath dex saves, etc.
its not as much of a unlikely situation as you present, So this spell maintains the level 4 or 5 spell you cast, which often requires an action or BA to cast, as well as dealing damage and reducing damage taken.
Thats without even considering agathys, or similar things. Like straight up logically speaking this spell would always be a better choice if its preventing a spell of 4 or higher from being broken that costs an action or BA, because itss maintaining the spell, reducing damage taken, and dealing damage, whereas recasting that spell without doing it costs more time, and you take the damage, you deal no damage, and has the same spell cost.
Especially if you are a mid/close range mage, this is a valuable spell, as you are bound to take hits eventually.
And remember, every spell is not for everyone in every usecase, the vast majority of spells you select are situational.
Thanks for the info!