Also “had slayed” I think should be “had slain”. Here and also at “slayed Slytherin’s basilisk”.
It seems “slain” and “slayed” are both valid as the past principle of slay, though I did have the same thought before I looked it up.
From the linked article:
The alternative past tense and past participle form “slayed” is most strongly associated with the slang sense, “to delight or overwhelm”:
In recent use, “slayed” is also often found associated with the other senses as well. However, this is widely considered nonstandard
Also “had slayed” I think should be “had slain”. Here and also at “slayed Slytherin’s basilisk”.
It seems “slain” and “slayed” are both valid as the past principle of slay, though I did have the same thought before I looked it up.
From the linked article: