How about something like , he can use his reaction to reduce an attack with the , , , etc , damage type by 1 dX damage , on his next attack he may deal that much damage of the type absorbed . Like reducing it by d 6 , then every few levels , you up the number of d 6 . Or increase the size of die like bardic inspiration . Maybe limit it to a number of times per day equal to his Con bonus to prevent it from being used on every attack . So , say he has a CON bonus of 3 , he could use this feature 3 times a day . He gets hit by a fire spell and reduces the damage by 1 d 6 and rolls a 4 . Next hit he deals an additional 4 fire damage . I also advise applying the damage reduction Before resistance if he gets resistance to Cold damage , for example , you reduce the damage then half it . For example , getting a DR of 6 on a 20 damage hit would work out at either 4 or 7 damage depending upon which was applied first . The latter seems to be more fair . Just be careful about how manyhow large the damage die are for the reduction , and the number of times he can use it , try to balance it based upon whatever feature you ' re replacing with this so the combined reductiondamage reflected work out the same as he ' d be able to do with that feature . I ' d err on the side of caution as it ' ll be better to increase the damage later than to try to reduce it the player is likely to be discouraged if you nerf him , but not if you buff his new feature .