Canonically Viekang survives and either kills Gorion’s Ward (Abdel Adrian) and becomes the Slayer or Abdel kill him and becomes the Slayer. Either way, the player party in the campaign Murder in Baldur’s Gate kills the Slayer, and this kicks off the events of that campaign.
I‘m hoping that none of this is mentioned in BG3 so I can just ignore it.
Amen