Adlet

While for the most part Dietrich seems pretty damn good, the bit with this Noble Phantasm is a little puzzling.

Nagelring
Giant-Slaying Blade

Type: Anti-Giant, Anti-Unit
Rank: B
Range: 1
Max. Target: 1~2 people

The very first sword given to Berserker by Elbegast who had it stolen from the giant Grim. It is one of his primary Noble Phantasm that he chooses to dual wield with Eckesachs. It is a beautiful blade forged by Giants and meant to be used by them, but Berserker had turned it into something else entirely. Due to Berserker's actions with this blade - which is the slaying of many Giants with their own forged weapon - this blade has morphed into something different than what it's original creators had intended it to be. Having been coated with the blood of the Giants, the sword has morphed into somewhat of an effective Giant-killing blade.

When he summons it as a Noble Phantasm, this sword works as a potent poison against heroes who are connected to Giants. In particular, the stronger one's connections to Giants is, the more potent the poison of this blade becomes. Because of this, it would be the best anti-weapon against someone like Fionn Mac Cumhaill who anecdotes of being a giant in legend or perhaps against Darius III who was claimed to be a giant. Usually, it would be impossible to wield this blade by humans due to it's humongous and heavy nature - after-all it was forged for giants who's strongest is far above the level of measly humans. The very fact that Berserker can lift and wield the blade of the giants is proof of his strength.

Unlike his other blade, this sword does not have a sharp appearance. Rather it's a very blunt and heavy blade. As such it will be used by Berserker to smash anything in it's vicinity to pieces. The force of the blow becomes more heavier against Giants. Getting close to Dietrich would be a foolish idea because taking the full force of the blow by the blade of the giants is anything but bright and yet at the same time, due to his dragon breath he becomes incredibly hard to defeat even if you stay out of the blade's range - for he will breathe fire to make up for the lack of reach.

This really doesn't sound like a B rank Noble Phantasm. A large sword that rises exponentially in effectiveness in terms of destructiveness and damage dealt against those with giant kinship?

That's E+, maybe D+ at best. It's worthy of a Noble Phantasm, but it's not B rank.