Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 16 - 16-52. Sage's Tower

Betrayed and left for dead, Richard Blackwood—formerly known as the world renowned hero Astralis—awakens a decade later to find the world has moved on, and an imposter now wears his mantle. Disguised as a rookie in the Mage Guild, he quietly hunts for answers.'If my little sister is levelled above me, then what about everyone else? How much stronger have the bastards who betrayed me grown while I was dead?'Richard must grow stronger once again, and rise much higher than the S-Rank.'What the fuck? I almost die, and I get a power up?'Not harem.

Talented in Magecraft but useless in physical combat?

In novels with a stud there is always a Senior Martial Brother like this:

In the early stages of the novel, his innate talent is high, his cultivation is good, his appearance is handsome, and he has an intelligent, cute younger Martial Sister as a fiancée.

In the middle stages of the novel, he is caught unprepared and defeated by a young disciple (the protagonist) in the sect who’s hardly remarkable at all. From then on, he falls from his godly altar and loses his brilliance. Even his young Martial Sister leaves him for someone else.

In the late stages of the novel, he continues to turn dark, going down the Demonic Path and becoming the major villainous boss. However, the protagonist still ruthlessly defeats him and his soul is eternally sealed.

Once Qin Mo opened his eyes, he discovered that he had become this Senior Martial Brother.

- Description from Novelupdates

Moulded from rot and souls, Loimos is born as an undead, as a skeleton. An undead linked to the weakest category of its kind, yet, he has something no other of the dead have. He is pure. Follow the journey of Loimos as he fulfils his duty and hunts down the living with extreme prejudice and faces off with the hypocrisy of those who live in death. In a world thriving with life, magic and heroes, he stands as the solution and its end.