Isekai Maou to Shoukan Shoujo Dorei Majutstu Wikia

Takuma Sakamoto is an ordinary human who is mysteriously transported to the world of his favourite MMORPG, Cross Reverie, with the appearance of his own character in the game, the Demon Lord Diablo. The two young girls who summoned him, the pantherian Rem and the elf Shera, attempt to use a spell to make Takuma their servant, but the spell rebounds, and both end up with magic collars stuck on their necks, thus becoming his slaves instead.