Read Yin Zhi Shoumuren manga
The MC is a really poor student that loves playing games. One of his friends is a popular student detective that convinces him into helping with a murder case. After viewing the last moments of an old man being held at gun point, the scene forces the MC into a virtual reality game, dropping him off at a desert landscape with some pyramids. The MC goes with the flow, and the first thing he sees is a BOSS sand monster and a quest option. Like any MC, he expects to defeat the Boss to win, right? No, it turns out he’s the last Gravekeeper and he’s supposed to protect the treasure against the Tomb Raiders. That old man? He was his grandfather, and they meet in virtual reality.