I could work more on the myth section, but until the motivation to write a whole ass story strikes, this is what you get.
Domain
Contracts: The domain of Memteus is that of contracts, that is a formal agreement made between two parties and sealed via a binding reagent overseen by an impartial third-party, usually Memteus himself. This is typically blood in the case of most mortals, although depending on what the mortal is making a contract with, further action may be required. Examples include allowing one's flesh to be burned as part of an agreement to have limited control over the element of fire, spilling a drop of blood in a basin of water in order to call upon the power of the sea, and so on. These deals, like all contractual agreements between two beings, come with their own sets of consequences should the supplicating party renege, break, or commit some act that doesn't fall within the purview of the contract itself. At the most basic of levels, the supplicant themselves may be afflicted with some manner of curse or outright killed, though in certain circumstances those close to the supplicant may be harmed or killed and the deal rendered void. Especially if the contract was made to defend or otherwise save a life/lives aside from the supplicant's own. These consequences are stated outright to the supplicant in question before the deal is officially made so that they may be able to properly weigh the benefits and risks such a course of action will provide.
Base Form
Memteus base form, or the one he wears while walking the material plane at any rate, is that of an elderly man. One whose hair has already begun to recede from the crown of his head and whose hands have become boney and worn from age. In this guise he will often clad himself in garments reminiscent to those worn by a wandering monk or saint, with the only unusual feature to be found being a massive leather-bound book, for it is within this book that his contracts are made.
True Form
To say that Memteus has a true form is to do him a disservice, for in his basest of forms he embodies the very concept of contracts and contractual agreements. Thus one can never truly grasp his actual appearance, as there is simply nothing in existence to grasp - be it in the material or divine realms - save for an idea that defines what a contract or agreement is. If anything, one could describe Memteus in this state as being more of a presence, though even that is inaccurate to a huge degree.
Myth
It is said that Memteus will often wander the world looking for those in need so that he may bind them within the iron will of a contract. Why he does this exactly is not known to mortalkind, and the only explanation the god himself has been willing to give is that, "Someone somewhere will always be in need of a contract."