Is anyone familiar with an immortal that might present himself as a tall , pale attractive man with long, black hair riding a stag made of fire? I had a dream in which such a man appeared and I believe it was a deity showing himself to me to work with me but I’m not sure who it was. Any help is much appreciated. Thank you.

No, sorry.

What kind of location / background were they in? eg Forest, desert, ancient, medieval, modern etc?

Was the stag the deity, or the rider?

What colour were the flames? If white, that may be reminiscent of WA stag or Ki Lin figure of light.

I think nature spirits of power that ride a stag in the forest are a fairly well known pagan trope.

Maybe you'd find it useful to google all reference to immortals and stags, or gods who rode them, until you get a gut sense of what this figure's significance might be to you.

Just remember that the immortals can reveal themselves in symbolic ways, so I wouldn't literally presume he rides a stag or a stag made of fire. What you want to do if you don't have a deity come to mind immediately or find one fitting this exact description is break down what you saw - pale man who was attractive to you, stag, fire. Also take into account surroundings, etc. as Lewk pointed out.

Here's what I can tell you generally speaking - usually 'paleness' surfaces as a trait to denote they are of 'the fair folk' (not that all fey have pale skin - of course not, but you're possibly being led in a certain direction here or to Celtic/European lore). Fire as an element is symbolically tied to magic, but also the martial - we could be looking at a magician or war deity (or both). Stags are otherworldly power, lord of the woods - this is regal energy for sure. Again it's coming together to make me think of a fair king, a fey king (which would also make them mesmeric/attractive to whomever they please), or a ruler over the elements/seasonal changes.

When I began to read your description, before even getting to the real details I was thinking of one of the fairy Kings. The one that came to mind doesn't seem right for what you went on to describe, but it's worth mentioning how that came to me if it might lead you to an answer.
Gwyn ap Nudd is a Welsh ruler of the Otherworld I am familiar with and he is sometimes associated with the stag - to me he exclusively appears with long, dark hair, but his face is typically painted and he'll be wearing green. This is why all of the little details you can recall are potentially important in identification.

The first deity that comes to mind if you just look at the details though might be Cernunnos, that's a pretty obvious one. Cernunnos is also seen by some as an 'equivalent' to gods like the aforementioned one.

I must also mention that the association of fire in this way makes me think a lot about Slavic deities, Kresnik in particular - who is also connected with the stag.

These are all of the potentials I feel good about bringing to your attention. An intensely magical force of nature has graced your presence no matter who it might be, with the associations I am seeing.

I can tell you that you're bound to know exactly who it is soon, as when a deity calls to you and has captured your interest they are not going to stop at just one dream.