Melos it is.
It's a dried powdery substance, vaguely lavenderish in color (from I think the berries of a heathery-type plant that clings to the wind-swept Rhythnorian hills), that you whisk into hot water until it froths; it's got caffeine, though I can't remember whether it's strong or weak. Obviously, we'd have variants that are either at this point, but on what scale? (Brown diner water? Super-charged green tea?)

Other'n that, I'm at a loss. Anyone else?

What Kip Said
Yeah, Kip pretty much summed it up. To it I'd only add that before people started using melos as a recreational beverage, it was valued as a medicinal herb. The leaves of the plant have strong astringent and anti-bacterial properties and may even serve as a mild antibiotic as well. It is associated with healing, with reconciliation and recovery, and also with memory (and actually, thinking about this scientifically for a moment, that "memory" association may well be due to the caffeine! Unsurprising, I suppose, that Cholaeic magi have such a taste for it...).

Like heather, melos blooms in the late summer/early autumn. It is highly unusual in that its flowers and berries appear on the shrub at the same time (the fruits develop from the previous year's flowers and take the entire summer to ripen). Melos flowers are small, whitish, and not in the least bit spectacular. They do attract many bees, though.

Melos is also highly flammable. Regular heath fires are part of the natural cycle of melos heathlands, and the people who live in those parts of the world start them on purpose on a regular basis, both to keep the fires under control and also because older melos plants do not produce well. The shrubs are one of the first things to appear in a place that has been burnt: they can pretty much grow out of ash. This has also contributed to the plant's association with healing and recovery —specifically, with recovery from warfare or violence.

I have no strong opinion as to how potently caffeinated the drink made from the berries is, but given that it is a medicinal herb, I suspect that if you brewed it up too strong, it might well make you sick. Or maybe not. Depends on whether the stuff found in the leaves is also in the berries, I guess.

I imagine the berries in their pre-processed state as hard little things, covered in a glaucus-lavenderish powder. Rather like a cross between juniper berries and bayberries.

Thank you, that's excellent!
Thank you, that's excellent!