I don't know about Necropolis, but Elves completely crush Demons.
I'm attaching a file, a map, with heroes modeled after a real game played on the "Empire" map. The level, number of units, and artifacts are as accurate as possible. The only difference is in the number of racial skills - for some reason, the editor only allows three slots for racial skills, so I had to sacrifice one skill for the super abilities.
All the Demon neutrals are placed in front of the Elf so that he can add them to his list of sworn enemies. The heroes have a limit of 999 units in their armies. All the others are in the castles.

In the final battle, the Elf, Wingael (me), attacked, and the Demon, Agrail (my friend and opponent), defended in the castle. As a result, the Elf's super ability, plus four stacks of sworn enemies, left the Demons with no chance. Even the units instantly summoned by Urgash's Call died before they could even take a turn.

If you're interested, try winning with the Demon against a human opponent, because the computer somehow managed to lose.