I believe the Runic blade is far too weak in the game. As a tier 4 weapon, its base stats are reduced by 5 points of maximum damage compared to the standard benchmark (from 4–25 down to 4–20). Yet in exchange, it only gains an additional +1 maximum damage per upgrade level (from +1–5 per level to +1–6 per level), with no other bonuses whatsoever. This is clearly unreasonable.
Before reaching +5, a tier 4 Longsword will have stats equal to or better than the Runic blade. Even at +6 and beyond, the Runic blade is still not significantly stronger than the Longsword. In the most extreme case, with a +21 upgrade, it only has 16 more maximum damage than a Longsword of the same upgrade level. But even that advantage is hardly meaningful.
If I were to invest those upgrades into a tier 5 weapon instead, I could achieve much better overall stats. If I used them on other tier 4 weapons, such as the Battle Axe, I would gain more accurate attacks; if I used them on the Crossbow, I could build around dart-related playstyles, and so on.
In short, I believe the benefits gained by sacrificing 5 points of base maximum damage are severely insufficient. Therefore, I strongly suggest buffing this weapon.
Love the game!