Familiars are not an assumed default - magic users must seek them out and entreaty them to enter into the Familiar's Pact, the details of which the creature in question will dictate. Not any creature can be a familiar, but there appears to be no common trend amongst those which can besides some touch of magic about them. Not all can speak, and so the use of Comprehend Languages may be in order. Once the Pact is made, the MU and their familiar will be able to speak and understand each other.
Familiars have something to gain from the bargain - this much should be obvious. They are individual, intelligent beings and demand to be treated as such. They have their own agendas, which they may or may not share. Many will allow themselves to be used as magical batteries, re-usable scrolls - although there is no guarantee that the familiar will not use the stored spell to further their own agendas.
Due to the limited number of familiars in the world, a popular acquisition method is the murder of the previous owner followed by offering terms to the now-master-less familiar - familiars cannot break pacts, nor can MUs. Familiars tend to take a vicious glee in this, regardless of their stance on violence in normal contexts.
Familiars can be asked to do a variety of tasks, and will generally agree, although this will vary by temperament, and they will never follow through with tasks they feel are too dangerous. As mentioned before, they can cast spells stored in them, either on own their own initiative or at the caster's order. They have HD appropriate to creature.
The only common theme for a given pact is the permanent loss of d6 HP by the MU, to a minimum of one. General terms should be negotiated in-character, whilst more esoteric requirements can be rolled below.
Roll as many times as the familiar tells you to.
**edit - blogspot swallowed my table, hence less than useful format.
Other familiars you meet must be killed, or their plots discovered and thwarted.
Weekends off. The MU must not travel too far from the familiar, effectively halting movement beyond a day’s travel. Weekends off can be waived, but only in promise of next week off at the familiar’s choice.
They must not be watched eating, and will only eat incredibly specific foodstuffs which may or may not be easily acquired. The MU must provide this food.
The MU must remain celibate. If this is not followed, the MU’s lover will be found in chunks about the next adventure location. The familiar will not divulge information on how this was achieved.
Rituals must be performed to the familiar every full moon, often involving expensive perfumes and incenses.
The MU, and by extension the familiar, cannot enter sacred ground. If they enter said ground unawares, they have d4 minutes of uncomfortable itching before they burst into flames. These flames do not go out until put out conventionally, outside of the holy ground.
The MU must speak and write in verse – no prose can be read. Spells are pushing the envelope, but just about get by. The familiar will critique the poetry.
Whenever a tattooist is discovered, the MU must get tattoos of arcane symbols and of the familiar. The first 3 must be easily visible.
The familiar will drink the blood of the MU every night, reducing healing by 1 point every other night from constant cuts.
The familiar despises a spell, and completely bans the MU from casting it. Determine the spell randomly. The MU is physically incapable of that spell.
All familiar effects only last as long as the pact - barring the HP loss, which represents the binding of the MUs soul with the familiar itself. It is theorised this is not actually needed, but used as a method of keeping masters under control.
Familiars can appear in any form, although most prefer subtle, easily explained forms like conventional animals, travelling companions and the such. Others are highly unconventional and have varying difficulties being hidden, such as a disease-familiar or a huge fantastic beast. They have the dietary requirements (unless specifically mentioned in the pact) of a standard creature of their chosen type. The form cannot change.