Mythos is another version of Rare Artifact, but you don't need an actual artifact and it allows some additional kinds of improvements. In exchange for this increased power, the temporary aspect may 'charge' up its free tag when the magic is used — this backlash comes from unseen servants of darkness hounding you! You may use improvements individually, so you could use 3 single-improvement 'spells' in a session. This stunt requires a 'linked aspect' (see below).
1) Stunts cost 2 improvements (so 2 instances of Mythos Magic can provide up to 3 stunts, if you combine them). You have to buy prerequisite stunts but other Mythos magic you have used that session can count if it contains the prerequisites.
2) Creation costs 1 improvement per weight factor — you can create one thing for the session, so be wise about what you create!
3) Weird improvements (i.e. new stunts) are possible and should be modeled according to roughly similar stunts in the RAW and have appropriate stunts (existing or new) as prerequisites (if the new stunt is not a tier 1 stunt) — for instance, check the limitations of roughly similar stunts, like costing a fate point to apply to everyone in a zone, etc.
When you use a particular type of mythos in a session, you get a temporary aspect for that type of Mythos (like with Rare Artifact). Backlash is represented as a free tag on the aspect. You can avoid backlash by succeeding at a mysteries skill. The difficulty of the backlash roll is 2 per improvement. If you fail your roll, you can still avoid backlash by spending a fate point (or just counter it when an opponent tags it for effect). If you use that type of mythos again later in the session, the aspect 'recharges' its free tag unless you succeed at another backlash roll — but if no one has used the free tag, they don't stack; there can be at most one free tag available at a time. Fate points you spend to counter free tags are kept to be used against you on future backlash rolls (including countering FPs you spend to avoid backlash).
This stunt requires a 'linked aspect' which characterizes how the magic manifests. In my campaign, two PCs had Mythos magic, but it manifested very differently for each of them. One was a Randolf Carter-type scholarly mage, so his magic manifested kind of like Dr. Strange's and one had Aboleth Heritage, so his magic was mainly biological effects from his own body.
Temporary Aspect Examples
What is that intense keening noise?
They're watching me
Transparent film over eyes
Flashback to my parents' murder
Older than dirt (character appears to be have aged extremely)