Yeah. I think summoning demons and devils should be a big deal, but summoning modrons and slaadi should be an equally big deal. For that matter, so should summoning angels. My take is something like this:
- Devils: The easiest to summon, devils are positively eager to respond, and quite helpful. The deals they offer typically involve immediate assistance in exchange for a measure of long-term control over the summoner. Though devils have a reputation for twisting words, they seldom do so in these situations. They prefer to build trust to encourage repeat business, with the ultimate aim of bringing the summoner wholly under the devil's thrall.
- Demons: Also relatively easy to summon, demons require grisly sacrifices to lure them in. They don't make deals. A summoned demon will do as you say for as long as you're strong enough to make it submit. The instant you show weakness, it will turn on you. Demons also seize any opportunity to wreak havoc, and their very presence is a blight on the world, so they are best kept on a short leash.
- Demodands: Demodands can't be truly summoned, as they are tightly bound to the plane of Tartarus (also called Carceri). However, they can be called to take delivery of prisoners. If you have someone or something you need locked away forever, summon a demodand to take whatever it is off your hands, no charge. Just don't ask for it back.
- Daemons: Daemons (also called yugoloths) are mercenaries who demand payment in souls, and you had better be ready to settle your bill when they report in. Unlike devils, they don't try to build long-term relationships. They're much more transactional; summon the daemon, make your offer, it does what you want, you pay and it leaves. If you give them a loophole, they'll take it every time. Fail to deliver your end, and they'll take your soul to settle the account.
- Modrons: A modron has a job to do, and it's going to do it. If it finds itself trapped in a summoning circle, it does what it must to get out of that circle and back to work ASAP. That said, modrons also operate under a bewildering array of rules and regulations, which dictate what they can and can't do at any given moment, so the modron will almost always insist on reams of fine print to any deal. Summoning a modron is a bureaucratic nightmare for which very few wizards have the patience. If you do strike a deal with one, it upholds its end to the letter (but not necessarily the spirit).
- Slaadi: Slaadi can be summoned but not controlled. No bargain is binding on a slaad, and it does what it likes without regard to its summoner. On the plus side, they bear their summoners no particular malice--they aren't evil like demons or devils--so they can be a great way to start a diversion or generally sow confusion.
- Angels: Angels are the hardest to summon, and absolutely will not submit to control. Instead, they must be petitioned for aid. Worthy petitions are answered; unworthy ones are rejected; truly unworthy ones are punished with a blast of divine fire.
