Making good use of concepts and information presented in the Paizo book Mythic Adventures, this brief work crams a lot in - not just the titular Mythic Glabrezu (a thoroughly nasty character), but a collection of mythic abilities suitable for a range of evil and demonic creatures and even a discussion of summoning rituals including two worked out in full.
Whilst the glabrezu is an unpleasant creature to have around, evilly-disposed individuals are tempted to summon them because they can grant virtually any desire the summoner might have. Being evil, however, the glabrezu does its utmost to maximise the damage it does in the process. They are nasty enough in their normal form, but mythic ones are worse... the regular sort delight in turning the summoner's desires back on himself, whilst a mythic glabrezu prefers to spread its malice far and wide, causing suffering to as many people as possible. They are sneaky as well, once loosed on the world, they slink around in disguise seeking to sucker the unwary to make use of their wish-fulfilling services... and only later allowing them to discover the cost.
A selection of mythic abilities follow, which you may apply to any suitable creature. The one I like (if that's the word...) is Demonic Possession, which gives the creature the ability to take over, possess, another living being in a manner akin to a magic jar spell, only the living being serves as the 'jar' - and, horrifically, remains fully awake of what the demon gets up to when in control of his body! Ugh, but great potential for plot development.
The final part deals with summoning rituals. Mythic demons require specialised spellcasting and ritual to make them appear and (more importantly, at least from the summoner's standpoint) to bind them. Both researching and then performing the ritual involves time, expense and several skill checks... get just one thing wrong and potentially you have a demon on the loose! The material here gives an indication of how to devise such a ritual - both in terms of flavour and game mechanics - and two sample rituals (including one to summon a mythic glabrezu) are given.
It's amazing just how much has been crammed into a few pages, and it's all good stuff. Just the thing for those budding demonology hackers in your campaign world!