Harold Tessmann III
I Slayed a Monster that had come up on a Card. In doing so, I released another monster. The only space available was the space from the monster i just killed. Do i instantly replace and fight new monster now as i try complete remaining tasks? Or am i actually meant to hold off getting the First monster until i've resolved whole Adventure? If i fail to claim the whole adventure, i still claim the monster i killed i presume?
Reread the rules, page 17 in the English version. You only collect monster tokens as trophies at the end of the Resolution phase, before resolving the success or failure for the adventure, and you do collect monsters as trophies separately from succeeding or failing the adventure. If you kill the monster using dice, you leave them on the token until that time; if you use a spell that kills a monster, you simply turn the marker face down. So by the rules, you'll never encounter this situation, though if you kill a monster and fail at the adventure, which failure results spawn a monster, you could end up with that monster placed on the slot you just freed up. The next player to attempt that quest would then have to fight the new monster, as usual.
(I haven't seen the expansion; they may have added effects that instantly spawn monsters or otherwise invalidate my answer.)