Random thought of the day… I like the classic cliché of fantasy where the evil, insane or fool warlocks summons Terrible Things From Beyond© through a dark and dangerous ritual, usually losing their lives (and souls) when a small syllable is wrongly vocalized.
You could, of course, achieve that idea in Pathfinder through Incantations, but let’s aim for something both simpler and more usable by player characters – Summon Monster spells. Any summon monster spell can be casted now on its perilous version.
A perilous summon monster invokes either a more powerful creature (use the table of the next level of summon monster) or 1d3 creatures of its level (instead of only one). For example: a wizard casting a perilous summon monster II could invoke either 1d3 creatures from the summon monster II table (like 1d3 giant spiders) or 1 creature from the summon monster III table (like a dretch).
The caster must declare which version of the dweomer he’s using at the time of casting (i.e. caster that must prepare spell doesn’t need to prepare a perilous version in advance).
As you can guess, casting a perilous summoning is… huh… perilous.
It has the following special requirements and drawbacks:
I – The spell’s Duration becomes “Concentration (Special)”;
II – The character can only keep the dweomer’s effect for a number of rounds equal to half his caster level (minimum 1);
III – If the caster’s concentration is broken, the summoned creature escapes his control and tries to kill either him or whoever damaged it last round (intelligent summoned monster will usually try both tactics, if viable).
There’re, however, a few “cheats” to evade some of those requirements.
The caster can cast a perilous summoning with a Duration of “1 round/2 levels (minimum 1)” – without the need for concentration – if he either offers a sacrifice to the outsider or a portion of his own life force. Both options, however, are amazingly risky.
If he chooses to offer his life force, after casting, he becomes instantly exhausted and at the dweomer’s end he must successfully pass at a Fortitude Save (same DC of the spell) or fall unconscious. If this happens, the summoned outsider doesn’t go back to his native plane but escapes control and stays an additional number of rounds (equal to the caster level) in the area. Usually a neutral or evil outsider will then attempt to kill its summoner, while a good outsider will teach him a lesson (cursing or marking him, planting a geas, burning his grimoire, teleporting him, taking his magic items to the Outer Planes etc.).
If the caster chooses a sacrifice, it must be a defenseless good and intelligent living creature (for an evil summon monster) or a non-neutral magic item of equivalent caster level (for a neutral outsider). Good outsiders don’t respond to summonings made with sacrifices. The caster must destroy the sacrifice at the dweomer’s end (the last round of Duration) or the outsider will be set totally free (like a calling spell) and will use all his strengths to kill the summoner.