Hear me out.

The orcs, under Adars guidance build the tunnels to light the volcano by pouring water in it from the reservoir through the tunnels. The reservoir has a dam with an old tower or keep on it that keeps the water dammed up. Waldreg, using the sword, activates the self-destruct sequence or sollbruchstelle of the dam. So far, so good.

Now, the orcs are looking for the sword to activate the dam. It's a big mess and takes a lot of effort and so on. Why do they not simply tear down the foundations of the dam? Same effect, faster, and no need to burn down some random Southlander villages. No sword needed, just some orc engineering and muscle, both in ample supply.

Also, the sword seems to be handed down in Waldregs family for generations, and he does know that it is important but not what it is good for (activating the dam trap card). So one might guess who made the dam made also the sword (they go together). It is probably not for mordoring the Southlands and activating Orodruin because the tunnels are a later addition that Adar came up with for his terraforming (Arda-forming?) project. The question of the origin and purpose of the sword/dam combo remains open.

So this raises the question: Did Adar need the black sword of Waldreg at all if his plan to succeed?