As a new DM i'm not quite sure how to plan this out. In my most recent meet, and by that I mean my only one, with my player's they were staying in a house one of them owned in the first city. A few of them were asleep and three of them were awake in the middle of a conversation. The doppleganger cleric, who used to be an assassin, the half-elf fighter and the Warlord Warforged, who was sitting in the corner listening. The main villain of my campaign is the head of a organization named Nikolai.
The doppleganger used to take jobs from them before converiting to the church of Berronar Truesilver after a job. He wasn't quite happy about it and sent an assassin to not only kill him but a few others in the party as well. I wanted to make the assassin adept with some magic abilities to make them seem as a wandering qizard than an assassin.
Earlier before the conversation started I told them that they heard a rustling noise on the roof. The warforged went to check it out but but only saw some birds flying away. During the conversation the Cleric was about to reveal that he used to be an assassin to the warforged and the fighter to gain the fighter's trust. Unbenownst to them the swordmage genasi was also listening in. On impulse before he said he was an assassin I stated that lightning crashed through the roof catching parts of the room on fire, and that while they were trying to put out the fire they heard a rustling again on the roof.
I wanted to know if this was a smart idea to set out an assassination. Or whether I should try a different approach to try to hint that there may be an assassin after them, and why.