I think it's a good idea to understand how it would be perceived from a player's POV when they're being attacked by it without getting the specifics of the spell.
If you tell a player "The necromancer summons a wraith and it appears next to you, he makes a melee spell attack and deals this much damage." The first thought that would probably come to mind, and the first action, would be to target the wraith. If they can't do damage, they'll try banishing it.
I don't necessarily think HP is required or even a full statblock. Something like "It has a Charisma and intelligence score of 1, immune to Dex, Strength, and CON saves and has a Wisdom of 10."
But it's already way too complicated for my taste at this rate. It'll probably confuse the mess out of new players and probably won't see much use because of the difficult rules and 1-2d4 damage vs firebolt.