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Starting a how-Lysander-was-able-to-kill-Grimwald theory list:
He was able to kill Lord Grimwald because curse had a time limit and expired. The Grimwalds aren’t aware of this, so they keep killing each other because they don’t know that they don’t have to. (See “The Annals of the North” on Ao3)
He was able to kill Lord Grimwald because the curse is conditional. The father and son are capable of dying in other ways, but if they aren’t dead yet, it will come to pass.
He was able to kill Lord Grimwald because he’s so powerful, the laws of nature couldn’t stop him.
He wasn’t. Lord Grimwald was trapped in the Sea Globe. (See “The Curse of the Endless” on Ao3)
I am a truther for a lot of things, but my biggest truth is that Dagbert is agender. Why? If Lord Grimwald had no first son, then Lysander could kill him all day every day no problem. He/They Dagbert who doesn't identify as a man or son or boy but actually just doesn't care