Pretty sure this is I Became a Legend a Decade Later After Telling my Comrades to Leave Everything to Me and Retreat First.
The sorcerer Luck believes this to be the end. Besieged by powerful demons, he makes the noble sacrifice of holding back their onslaught while his friends escape. Never sleeping or resting, Luck endures the clash of swords, rain of blood, and never-ending stream of enemies. Against all odds, he vanquishes the commander and emerges victorious, only to find that 10 whole years have passed in the meantime. Furthermore, his body has regressed in age to the point where he looks like a mere teenager.
Following his triumph, Luck soon reunites with his friends, the hero Erik and warrior Golan Morton. As they believed Luck to be dead, the pair made sure he went down in history as a hero and prepared to quell the rise of the demons themselves, unaware of Luck's feat. With all this in mind, Luck just wants a laid-back life, so he applies as a novice adventurer under the name Lock and sets out on a journey alongside brand new companions.
The story begins with a warrior (Golan), a hero (Erik) and a sorcerer (Luck) in a cave, which is actually an Interdimensional Rift. They've just fought and wounded a demon lord called the Majin Ruler, which has since retreated. Before long, they're attacked by an army of lower class majin. Since Luck is a bachelor, while Golan and Erik have families, he tells them to escape while he holds the majin off. They do so, albeit reluctantly.
Luck fights and kills numerous lower class majin, and is confronted by a stronger majin, known as a Majin Commander. The Majin Commander uses a Life Transfer Draining Touch on him, but Luck has the power to learn any sorcery used against him, so he ends up reversing the attack, which replenishes his mana and health. He also claims the Majin Commander's sword as his own, and states that he'll be able to fight for years.
He fights for so long that he loses track of how much time is passing, and casts a Marionette spell on himself so he can keep fighting even while asleep. He's eventually confronted by the Majin Ruler, and as a result of draining its life force, his body reverts to the appearance of a teenager (he was thirty when he entered the cave). The Majin Commander's sword was destroyed in the battle, so Luck claims the Majin Ruler's sword as his own.
With all the majin seemingly vanquished, Luck exits the cave and returns to his city, but notices that it looks different than he recalled. He learns that ten years have passed, and that he's now revered as the legendary Great Sage who sacrificed himself to hold back the Majin Army; there's even a large statue of him in the middle of the city. It also turns out that Golan became the Guildmaster, while Erik became the King.
Since Luck looks like a teenager now, most people (Golan and Erik aside) don't recognise him, despite his legendary status. He doesn't want the fame, so with the cooperation of Golan and Erik, he decides to start over with the new identity of "Lock," pretending to be an F-rank adventurer, even though he he still has his S-rank abilities and the powerful sword of the Majin Ruler, which he carries on the back of his waist.
When Golan's teenage daughter, Celliss, sees how affectionate he is with Luck, she misreads the situation and assumes that Luck must be a bastard child of his. It's also revealed that Erik's wife, Levi, wields holy magic, and that she used to be a member of his party, along with Luck and Golan. She didn't go with the others to confront the Majin Ruler in the cave ten years earlier, because she was pregnant at the time.
