I've been wondering about this. As Keeper of the Balance, Leknaat doesn't like it when powerful runes end up in the wrong hands. And Neclord lived for 500 years doing lots of horrible things. He stole the moon Rune from Sierra's village and allowed its inhabitants, including children, to die of starvation or live on the blood of humans. He wiped out entire villages, turned armies into undead creatures, caused people to kill their own friends and relatives who had joined the undead, and forced young women to be his brides. And Leknaat took no action or had no say against these atrocities?
Were Neclord's deeds not important enough to attract Leknaat's attention? What's one vampire causing havoc in a world where people are engaging in bloody skirmishes in territorial disputes all the time?
Could it be that Leknaat was not even aware of Neclord's actions? Maybe while he was allied with windy, he was protected from Leknaat's sight and free to do whatever he wanted?
Maybe Leknaat encoluntered Sierra once upon a time and Sierra told her not to get involved in the affairs of vampires?
She doesn't even pop up any of the times we defeat Neclord to congratulate us or thank us. I mean surely by defeating an almost unstoppable foe from turning everyone into zombies and returning the moon rune to its rightful owner, we've restored a balance?
It could have been interesting to see how Leknaat and the Star Dragon Sword would have reacted to each other.