Hmmm. Yes, at your stature, with your constitution... An ounce of hezrou bile, two spoons of wyvern blood, and a corpse flower's nectar. That would do.
Rycke is in the Bhaal Temple in the Undercity of Baldur's Gate. He stands alone on the upper balcony of the temple and never descends to the central altar ground.
Rycke apparently prefers to poison his victims with elaborate concoctions and considers 'regular' murders messy. If a non-Dark Urge character or a companion speaks with him, he says he looks forward to creating a poison tailored to them, or – if spoken with as the Dark Urge – to watching the duel between them and Orin.