This is a CRIMINALLY underrated game. I bought a physical copy of it from a market, not too many years after it released, and have played it to completion over and over.
The Eternal Night is an action platformer, with gameplay focused on attacking enemies and trying to get a high "combo". The home console versions of the game played a bit like God of War, so this in turn is like a 2d version of that and it's very well executed. Spyro has some basic melee attacks, but depending on the moves the player mixes in, the attacks will eventually leave the enemy time to attack, or knock them backward to break the combo. Higher combos reward the player with more gems to spend on upgrading Spyro's signature Fire, Earth, Ice, and Electrical attacks. So the main gameplay loop becomes progressing through each level, trying to achieve long combos on each enemy by mixing elemental and melee attacks. The system takes practise, rewards creativity and skill, and keeps the combat from becoming "just like all the other platformers".
What I really like about this game is that Spyro's abilities are unlocked gradually over time, either by collecting blue gems or by collecting these gold relic objects that are found in certain locations. It's good motivation to explore each level thouroughly, and there are even a few that can only be obtained by revisiting a level after other abilities are unlocked. Almost every ability can be toggled on and off too, so you can create your own challenges by limiting what is available. Some of them are a little too specific though, like the rolling attack that only works on the games few steep slopes. I think the only time I used that attack was on the slope nearest to where it is unlocked, and even there it's not very useful. Despite this, it's always nice to have a large number of abilities to play around with, especially if the controls are simple.
Aesthetically this game is brilliant. It looks great both on a modern screen and on the original unlit GBA screen. As one of those games released late in the GBA's lifecycle, it actually has the option to change the brightness of the graphics, which is a big help. The colours really pop out, and there's a lot of attention to detail in the particle effects and animation. Water ripples and splashes, lava bubbles, and little insects crawl around the natural scenes. The only other (older) game I think might be comparable in its level of environmental detail is Rayman. The music in this game is really great too - moody, dramatic, calming, and whatever else it needs to be to fit each level. I used to stop in some areas just to listen to the music and watch the animations for a bit. Thankfully The Eternal Night has a map screen accessible after clearing each level, and you can re-visit levels as you like.
Overall The Eternal Night is a high quality game and definitely one of the best on the GBA. It's up there with greats like Metroid Zero Mission and Wario Land 4, and I highly recommend it.