I could be saying this because I started to listen to linkin park when I was 11 and it altered my brain chemistry permanently, but I don't mind the tightfisted, blatantly edgy prose... once you start reading between the lines here, there's something genuinely engaging and unique that mainline fire emblem has not touched before or since and I think it's wonderful.
It really doesn't matter to me how "good" or how "bad" the gameplay is, it's a technical marvel for its time and I appreciate it regardless. The Last Promise is just something special and it's something I would point in the direction of fire emblem fans looking for something more

I don't know amigos I loaded an eight year old save state because my ass was five chapters away from ending the game and I farted through it. Due to this elongated time period between play sessions and my desire to skip cutscenes I can't tell you what happened in this shit outside of the lengthy cutscene that played as Anakin Skywalker and Kelik (killer spelled backwards) shared a tender moment with the final boss dork I triangle attack'd twice into sweet oblivion. That cutscene may have been unskippable but because of ye olde holding space on visualboyadvance I could see the endless dialogue scroll in hyperspeed while my eyes glazed over waiting to see how hard I fed Lirin. What a moment in time not just for the fire emblem romhack community, but for every Hulkamaniac (have fun with my family and friends).

Mandatory to listen to this on loop while reading

The Last Promise is one of those rare fangames with such a legacy to it that it might as well be counted amongst the official series. To the casual observer, that legacy is just the scene where the protagonist gives an edgy speech about handling the weight of death and kills a bunch of knights while a Sonic 06 midi plays. But to those too far gone into FE communities, it's seen as an technically impressive feat that, especially for a time where FE rom hacking was in its infancy and even more especially when you realize it was made primarily by a teenager. It is not an understatement to say that the current FE hacking scene owes its existence to this game. Playing it and checking off other FE games you unironically prefer it to in the same way a military sniper checks down their kids is a time honored rite of passage. I've seen a fair bit of people echo the sentiment that it's better than all three of the official GBA FEs and while Sacred Stones has too much personal significance for me to fully agree, I absolutely can see where these people are coming from.
Mechanically, it doesn't do anything particularly unique among other FEs. It's just a really good example of the enemy phase focused gameplay associated with 7-9. At its best, you get creative maps like Chapter 12, a Thracia style escape map balanced around having limited resources at your disposal, Chapter 5x, the rare defense map that's actually good, and Chapter 27, a map with gimmick so good that Engage may or may not have ripped it off a decade later. At its worst, you have to deal with a couple long and boring maps which are nowhere near the worst ones official FE games have churned out.
Narratively, it's a mix of genuinely cool stuff and stuff that's ironically funny. Whereas most pre-3DS FE games try to use prose that fits a medieval setting, The Last Promise is written with a bluntness that feels "online" for lack of a better term. This results in a lot of scenes taking on a more comedic tone than intended: the infamous Sonic 06 midi speech, the earlygame chapter where Siegfried gets mad at some random village for not having a milita in a way that can only be described as the former being portrayed as the Chad and the latter being portrayed as the Soyjak, and a major boss who's so edgy that his description reads "Plagued by a hatred for anything that lives, he is 'the Blood Reaper.'" But still, there is a fair bit of genuinely compelling stuff here. The prevailing dynamic between Siegfried and Kelik both being bound to a promise they made because of the death of a respective loved one but the former being an idealist who carelessly rushes into battle and the latter being more aware of his material conditions and working within them, at the cost of having a hard time trusting most people. Both learn and grow from each other as seen in the opening of Chapter 23 and a subsequent optional talk conversation where they get genuine and emotionally vulnerable with each other and their dynamic with each other culminates in an awesome climax. It's nothing groundbreaking but it's impressive for a teenager and I'd put it above most FE stories (low bar admittedly).
However, what really makes Last Promise stand out as special isn't so much any individual aspect mechanically or narratively but rather the end of an era it represents. The patch completing the main story was released on August 17th, 2012, just four months after the Japanese release of Awakening and six months before the Awakening release of said game. The Last Promise feels like a time capsule of when FE was seen as this niche thing that you played for the mystique of a game where characters can actually die and not a mainstream seller of millions of copies that you played because you wanted to date and/or fuck the characters. There's so much little things that give me anemoia for FE communities I was never in, be it Siegfried's personal weapon Nothung harkening back to a time where calling a unit a "mage-killer" wouldn't cause you to get laughed out of the discussion, the character portraits being collaborative effort by multiple FE forum users, and the use of bit crushed OSTs of more niche RPGs like Valkyrie Profile and Ys hammering that this was when most people saw FE as just like those games. Playing The Last Promise a decade after its release gave me a feeling of discovery that I haven't had with new FE games for a long time.
Is The Last Promise a masterpiece either mechanically or artistically that's worthy of a 9/10? Probably not but review scores are bullshit and shouldn't be seen as anything grander than "I like this game this much." The Last Promise just has an indescribable aura to it that transcends traditional quality. Maybe being an FE fan for this long has done too much damage to me mentally and these paragraphs seem incomprehensible to normal human beings idfk.