Since this came to you, unbidden, and you have not verified identity, I would first all all be very leery that this is an incubus impostor merely looking to eat your energy.
Your first priority should be to verify this entity is who it says it is. This is basic protections 101: It’s VERY unusual for a higher being to get sexual with humans, especially without consent or a basic introduction or asking - they DON’T need this energy. These are normally parasites in the astral, so verifying is a worthwhile activity.
Marbas is a higher being, there’s no reason he’d target you for a sexual relationship out of the blue. But every reason a local astral parasite would.
This is your first mistake. NEVER rely on anyone else’s UPG, especially not “mediums” who are totally untrained in the difference between subconscious impulses and conscious mind interpretations. If someone call themselves a medium, run. Not a word they say is not filtered through their own personal lens which has nothing to do with you. These people LARP for a living, happily unaware they’re living their own fantasy. There are ways to verify this info, and this is not it.
Without knowing who it is or if it’s an old friend? Don’t do that. Approach all unbidden, uninvited entities with suspicion. Do you want parasites? This is how you get parasites. Don’t be a gullible mark. Stand in your power, make you own decisions and find out for yourself first.
Big red flag. This subterfuge is not ok, You should have sen sings that Marbas was inviting you to make conscious contact. It’s more likely this is fake and it’s playing on your human desire to be special.
Bullshit. We’re ALL old souls or we wouldn’t be good enough to be human. This is obvious flattery designed to lower your guard.
Fake. There’s NO need for non-human incantations when your native language would be more useful and meaningful for you. This is a lame attempt to impose a sense of mystery and authority. The real Marbas doesn’t need to play this kind of petty head-game.
Yes another red flag. The truth does not mind being questioned.
My advice, if you want it, is to banish and check and set up proper wards and shields, as BASIC precautions that all mages should have.
Nothing should be able to come this close into your space you randomly without being specifically and personally invited. Anything else is astral "random’ and highly suspect.
Your protections FAILED - fix them, regain control of your space and your energy as a mage. Not a pathetic “medium” who is tossed without real understanding on the waves of the astral, an occultist that controls and understands what the fuck she’s actually doing, which most natural “medium” never, ever do.
Hell no. Who is “me”? You have no idea. NEVER believe the words of potential parasites, don’t even bother talking to them - they only lie.
For you are yummy and taste good with ketchup. That in itself is good - you are developing well and shining on the astral attracting attention, but you HAVE to protect your energy better than this,
Bullshit meant to wow you and make you feel inferior, distrust yourself and give away your power to the parasite…
After you have fully and properly cleansed your space of bullshiitters, seen and unseen, then call Marbas specifically, check identity, THEN ask him - knowing that it’s actually him - if that was him and what he was talking about.
Then you have a chance of getting somewhere real. It’s mistake was approaching you sexually first. Massive tell that it is basically only interested in eating your energy first.
if you are unable to evoke and ask Marbas directly, give the Unknown a task - ask it to perform a magickal but needed action in short time. Parasites can’t do anything real, they promise the sky, flatter you, give you fun fancy titles, but never deliver, and you will never know their actual name. They take and do not give. And no, getting off doesn’t count. 
Sex sells for humans, don’t be fooled with cheap marketing, the longer you stay attached the weaker you get and the stronger it gets until you can’t get rid of it without enormous effort - a bit of care now will save a lot of problems later. A parasite can suck your entire life dry until you’re friendless and homeless, nix that in the bud right now.