I think you're treating a "might" clause as a definite claus here. There are a lot of beings in Lovecraft Mythos that make deals to gain power. While there are many options available, it is incorrect to say that an intentional pact would not be applicable. It very well could be. Again, this is one path specifically allowed in the PHB, but not the only path ... and when one path is labeled as something that "might" happen, I tend to think the other option would not also be labeled "unlikely". Might usually conveys a lower probability rather than a higher one. When as say something might happen, it is usually to dissuade someone from believing it is impossible. If it is likely to happen, we'd tend towards words likely likely, probably or certainly.If you look in BG3 you can see clear examples of
beings making deals with Far Realms inspired entities to gain power, safety or other boons in exchange for service.
Londo Mollari of Babylon 5 would be another example of someone that makes a deal with an Elder God type in exchange for power. I used that as an inspiration for a PC early in 5E that died too quick - He was a scholar that was infected by a parasite that was the actual one performing the magic. The scholar was a normal mortal, essentially. The thing would compel the scholar to do what it wanted him to do if the scholar was not compliant. The DM and I had fun with it briefly... when the thing compelled the scholar to do stuff, I'd leave until the DM called me back so that I would have no idea what took place while compelled. The PC, unfortunately, took a critical hit from a giant when he was low on hps .... and went straight to negative max hp ... sigh Personally, I think the Shadows are really obviously Fiend-type beings rather than GOO. They aren't even slightly incomprehensible (nor are the Vorlons), they just couch all of their really quite plain motives behind seventeen unnecessary layers of metaphor. There's always a clear reason why they're doing what they're doing, especially when it comes to Londo. (Indeed, the fact that their 'avatars' are reduced to practically child-like dialogue after getting told off by Delenn and Sheridan--"Will you...come with us?" [...] "Then...we will not be alone?"--shows to me that, for all their metaphor, their desires remain very comprehensible.)The closest example (of media I actually know, e.g. I've never read Bartimeus books, I never watched Buffy, etc.) mentioned above is Zorg from The Fifth Element. Even there, the horrible thing, "Mister Shadow," comes across as being quite aware of its servants and, again, having pretty straightforward, albeit evil, motives. It wants to destroy all life, and is cunning enough to manipulate others into helping it achieve these aims. However, it only speaks once, generally is beyond comprehension other than "absolute evil," and has weird powers that don't really seem to be traceable to a physical cause (the "black goo" stuff, mostly.) So you could argue that its appearance of understandable-ness is simply because it's a particularly malicious eldritch horror, a la Hastur, rather than because it really is all that comprehensible to us.
Outside of outright Lovecraftian stories (or the seeds from which his work grew, like Robert W. Chambers' reinterpretations of Ambrose Bierce's Hastur), however, it's pretty rare to find things that truly fit the mold of Great Old One type beings, rather than the other two main patrons, Fiend or Archfey. Alien beings prior to the 19th century tended to be fairies, which while certainly alien were not that kind of alien. Some of it, I suspect, comes from the flowering of scientific understanding in the 18th and 19th centuries, but some of it also comes from the ennui of the industrial era and the horrors of 19th and early 20th century warfare. Some likely also comes from the long-term hegemonic status of the Abrahamic religions in Europe and the Near East and the "god is dead" stuff that had cropped up in the 19th and early 20th. In prior millennia, you could have your (good!) gods and the enemy's (evil!) gods, but that just gets bound up with the Fiend pact now (note where many demons'/devils' names come from.) Great Old One stuff is of a different caliber--the yawning void itself, given sapience. An entity that truly would prefer to destroy everything, as opposed to wanting to topple the existing order and replace it with a distinct but still existing order.
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