They didn't evolve at all, because they they aren't living creatures per say. They are extremely advanced machines from a forgotten past.
So before we begin, let's lay out what we want these creatures to do: according to your requirements they must be able to:
be able to keep track of all spells/magical abilities acquired, what they do and how to use them.
essentially human level intelligence in order to be able to not only use these abilities efficiently in their most basic form, but also to be capable of thinking outside of the box, coming up with new unconventional ways to use and even combine said abilities in order to achieve its objectives (despite the presence of other animals which are highly intelligent, I'd say humans are still the best example around of a creature with all of these, that's why I classify it as human level).
enough confidence, essentially to the point of absence of doubt, that it's capable of performing these tasks without flaw in order to be able to efficiently manifest these abilities.
But what if these creatures weren't actually something that evolved naturally? What if they were made, BUILT by someone else who needed such a thing to exist?
You said that Alendyas is a world present within a fracture of reality that's constantly growing and absorbing new worlds. However, if it has always been absorbing worlds, one of them had to be the first (warning, the following concept contains high levels of cliches regarding magical worlds with ancient civilizations, please bear with the lengthy text).
Their origin is the following: one of the first worlds absorbed into Alendyas housed a species of highly intelligent beings much like humans in overall anatomy and physiology. While these forerunners (as I'll refer to them) didn't have a solid understanding of magic or any remarkable ability to manipulate it at first, they were incredibly advanced in the field of technology, and skilled in the art of robotics and artificial intelligence.
Being now trapped in a world filled with creatures which at first would seem pathetic, but later prove to be very troublesome, if not lethal, as well as with a new, never before seen source of energy just existing all around, it'd be no surprise that these forerunners, being the only group of sapient beings around and with plenty of time after settling the proper fortifications, would immediately turn their attention to magic and start studying it, it's workings and how to use it to their advantage.
Over time, they came to understand exactly how enchantments worked, how to exploit them and how to control the ways these would manifest, but there was one problem left: despite understanding magic and the enchantments attributed to its existence, they were almost completely incapable of harnessing it themselves, a problem they would bypass through the creation of constructs.
Constructs were specialized machines, similar in many ways to pieces of armor, meant to serve as an extension of the forerunner, absorbing magical energy from the environment and transforming it according to the user's will. These became a part of the lives of many forerunners, with the exception of those revolted at their new life and contrary to the uses of magic. These constructs, at first simple pieces of gear with only basic AI meant to perform mostly simple tasks, became more and more advanced as time went by, until the point when the first E.M.E Unit was made.
The E.M.E (Exploration and Monster Examination) unit was a particularly advanced construct with a clear purpose: to learn new information about the world and pass it on as well as to study the many monsters that existed. It was, at its core, a conglomeration of specialized nanomachines controlled by a complex AI capable of abstract thinking and advanced problem solving capabilities, as well as top notch magic conversion technology. Thanks to that, and the fact that it was essentially powered by magic, it was a being to be reckoned with, being incapable of becoming tired so long as there was magic for it to absorb. Even at its most basic level, it was already fairly powerfull, with its core mechanics being:
A process similar to digestion in order to acquire matter and base materials to replicate its nanomachines and repair damages sustained on its own, eliminating any need to return from its exploration mission for repairs and reducing the risk of destruction of the unit.
the ability to morph its body into weapons (for self defense) and tools (for collecting samples for analysis), as well as to alter its body shape and morphology into whatever form it deemed most efficient according to its surroundings in order to efficiently reach the locations it deemed as worthy of exploration.
the ability to absorb and control magic, using it as a power source, as a complement to its "digestion" and self repair mechanisms as well as for self defense in the form of simple attack spells.
Due to its innate knowledge of magic and ability to manipulate it, it was also capable of controlling the enchantments acquired from monsters it killed, manifesting them according to its needs.
Despite its might and potential in deepening the forerunners's understanding of the world, only one of these was ever made, due to the sheer amount of resources necessary to build a single unit, a great problem in a society with most of its infrastructure regarding the harvest of metals and other key components for technology being very limited.
As a result, this souped up research drone worked for decades as the main source of information to the forerunners (which mostly remained in their city), learning and updating its programming and acquiring new knowledge, which it'd constantly relay to its creators. the many enchantments that it learned via various processes via trial and error, from consuming dead monsters to simply killing them to others, were also incorporated into its arsenal and database. Overtime, it learned how to use its repair ability and magical power to essentially reproduce, creating copies of itself as a means to cover more ground and to prevent the end of its mission due to unforeseen circumstances such as its destruction, at which point the units began transferring information to each other as well as to its creators.
However, there's a reason the E.M.E unit is one of the only remains of the forerunners, and why their makers are nowhere to be found: The appearance of ideologies about some individuals being better than others, fights for power and disagreements with groups that were against the use of magic resulted in the rise of a gruesome Civil war, which destroyed most of their main civilization and drastically reduced their population, as well as causing them to separate into smaller isolated communities. These communities, now much weaker and incapable of recreating most technologies due to the loss of archives and destruction of supply chains for the necessary materials, were slowly wiped out by a mix of monster attacks, genetic issues related to inbreeding over several generations and extermination operations carried out by the E.M.E, being finally wiped out from history along with most of their knowledge and technology.
As for the E.M.E themselves? They're fine and still active. Their creator, worried about the dangers that the forerunner technology could be to any future societies, entrusted the world to them, sending 2 last commands to the best of their creations right as the war began: " Defend your existence at all costs, and eliminate all traces of [forerunner's actual name] presence possible.". Now they roam the lands performing their primordial objectives: to learn all there is to learn, and see all there is to see.
After this overly large, but (as far as I see) necessary origin story, I'd say the E.M.E check out all of your demands for a" focused" creature:
they have basic human level intelligence as well as the abilities to solve problems, be creative and think outside the box.
they are designed to sense and use magic extremely well in order to carry out their primordial tasks.
they have a strong enough will as they fully trust their programming and skill to manipulate their abilities and acquired enchants efficiently in order to achieve their objectives. They could potentially be able to make these decisions much faster than an organic creatures could as well.
-it has an actual origin story on how it came to exist that isn't "one day it started existing out of nowhere and everyone just went along with it".
So yeah. My answer to your question? "Nanomachines, son!", except these ones make up an advanced research/military superdrone from a forgotten civilization. It can technically digest stuff, it does have a protocol ("""instinct""") of self preservation and it can technically reproduce if they gather enough matter for it, so it's understandable why it could be taken as a very weird monster instead of a robot (especially when you know what a monster is, but not what a robot is, at which point you just call it a metal monster and start running).
Now, if the creature has to be an actual organic being at all costs because the plot calls for it. Well they were extremely advanced, what is to say they weren't also very good at genetic manipulation couldn't simply modify one of the existing creatures to boost its intelligence and magic prowess/sensitivity? I'd not advise to give a species human level intelligence if you want them to be slaves forever though, unless you have a way to ensure they all have a thing for serving your species or something similar. Either way, it stopped serving, is out there now and helped with the forerunners' extinction so there are currently little to no traces of their presence in the past (unless you want to give your humans some cool advanced weapons without leaving the overall medieval tech level scenario).