I love the vibe! I don't think the vocals are bad at all, you used your voice very well for the needs of the track. Overall, I think the track is maybe a bit long without a clear hook. I'll try to break down my thoughts:
Melody Clarity 4/5
I have no problem picking out the melody here, the vocals don't necessarily cut through but it fits the vibe of the track. At least I think I understand what kind of a feel you were going for here.
Melody Impact 3/5
The melody is good, but for a five minute track I feel like it never really hits as hard as it could. It's kind of missing a hook, that one impactful part that really sticks with you.
Composition 3/5
This relates to the earlier point. The track is quite long, but doesn't necessarily justify it's length. The feel of the track stays very similar throughout, it could benefit from some contrast or a surprise?
Listenability 4/5
I have no problem listening to music like this! As I said, I like the vibe and would listen to this kind of music anyway.
Correlation to Theme 2/5
Just listening to the track out of context, I do not hear the correlation to either the painting or the overall theme of a legendary weapon. With the lyrics and your description it's a bit better though, but I still don't fully hear it.
I think you have a lot of skill for music like this! The main thing I'm missing is more.. dynamics? I guess. Some variety in the track, where it's sometimes more full, sometimes empty, sometimes quiet, sometimes loud. More clarity between different sections. You already had some ideas like that with the track rising in intensity towards the end, that was well done!
Keep up the good work! Hope you release your EP one day.