Next session the party is (hopefully) visiting the tomb of a giant, only to find a necromancer trying to revive the giant. So far, my encounters have been quite straightforward, so I want to spice things up a bit. Therefore, I had the idea that the person doing the the ritual is only a vessel and the real necromancer is somewhere else. I suppose this could work with simulacrum, but I'm not sure if it really makes sense, as the duplicate relies on the creator's commands, who (in this case) is far away. Is there any other spell or an object that I could use instead?

There’s sending stones, but they only work 1/day. You could always say this wizard has a better version that works unlimited times a day. Or if the wizard only needs to tell the other person to start the ritual, instead of giving step-by-step instructions, once a day could work, too. Though at that point, they could just use the sending spell. 

Kind of sounds like the Magic Jar spell, but with the range boosted. Or perhaps a twist on a lich's phylactery. 

Keep in mind that your baddie doesn't need to play by the rules. If you want the party to fight an animated statue containing the spirit of necromancer, or a sentient mop controlled by the necromancer's vibes, you can do that. Put another way, if you can't find a spell or item that does what you need it to, you can make one. 

The obvious answer is astral projection but I dont know if using player mechanics is a good idea it invites replication and often has complications designed to prevent player abuse. You have to be allot more careful than with an npc only mechanic. 

Of course you can simply say they based a custom spell off another spell and let players learn the normal version.  Saying something along the lines of " some of the principles of this spell seem incoherent or beyond you but you can develop a working version in the form of ..." 

astral projection  can be made more interesting for example by instead of having him appear simply as a wizard he appears as something that represents him which may be monstrous and corrupted due to his nature 

Alternatively like suggested he could have a modified magic jar which he uses to posses some one and make them sacrifice themselves in a ritual.  A sentient magic item can also control people similarly

What level is the group? You could always make the "necromancer" be a humanoid being controlled by an intellect devourer. LOL

If it's a Necromancer as the real bad guy then change the Simulacrum spell. Instead of casting a replicant of himself, he can imbue dead bodies to be his Duplicate. This gives it more flair that ties well to the theme. The villain will be puppeteering the undead creature so it is as if he is there but he will have no care for his well-being as the undead he is controlling is only a vessel. This will also allow them to interact with a possible reoccurring villain who will get to know them as well. You could repeat this several times throughout the adventure if you wanted. Thus making the defeat of the Necromancer more satisfying. Yeah, it's kind of Doctor Doom-ish, but why not? We pull from other sources all the time.

For the mechanics of the story, caster types are always making new spells. Or tweaking existing ones to fit their fancy. So making a spell similar to the Simulacrum but using undead as the body would work well in my opinion.

who cares about RaWW, just homebrew a magic item to fit your needs, you are the lord and master of that world after all. a mask put on the simacrilum that acts as an earpeice, ignore the 1/day propertiy of the sending stone, the sky is the limit

Just a Dominate Person. 5th level spell, unlimited range.  Wisdom save, so dominate a Wizard and make them do the casting.

You could use the 8th lev necro spell, clone. The tomb comes apart when the giant is revived (maybe the giant accidentally kicks a wall and the ceiling falls), killing the necromancer...but his clone revives somewhere far away. The giant is easily strong enough to get out of the ruined tomb.