NAF – New artifacts with unique properties.

The ring already exists, but it is extremely rare.

Like any other relic, including the "racial" ones.
By the way, keep in mind that when new artifacts are added to the game, the probability of obtaining any specific artifact (including old ones) decreases.

The thing is that in Heroes 5, the "War Machines" skill is not a priority, but a secondary one; it is rarely developed, and players prefer to focus on developing luck, attack, defense, magic, etc. This skill is essentially weak compared to the others, and one could forget about it, ignoring the obvious imbalance, BUT:

This is a personal opinion; players of TE, for example, will certainly not agree with this, because "War Machines" is a key skill to avoid losses in siege attacks (tents), and that is very important.

It is unlikely that any new artifacts can be added to "War Machines" because they cannot be directly affected by scripts (in Multiplayer). By the way, "War Machines" (ballista) are indirectly enhanced by any artifacts that increase attack and knowledge, and they should also be enhanced by a quiver (+4 attack for ranged units).