Here are two idea of how Jon Snow/Vimor Targaryen become the Overlord
1) He find the ruined fortress of one of the fortresses from the previous Overlord games and meets Gnarl who in his charming evil self put him on the evilest path of conquest and domination
2) The spirits of the previous Overlord (Black Baron, First, Second, Third, Fourth and Inferna) visit him in his dream and inform him of his destiny and convert him into their future successor and champion to their cause
Also do you think I should make the Dragonborn for the story
1) Jon/Vimor
2) Ariana/Vaerla
3) Original Female Nord Character