This sounds like a textbook case of surprise.
In general:
- Determine surprise. The GM determines whether anyone involved in the combat encounter is surprised.
- Establish positions. The DM decides where all the characters and monsters are located. Given the adventurers' marching order or their stated positions in the room or other location, the DM figures out where the adversaries are — how far away and in what direction.
- Roll initiative. Everyone involved in the combat encounter rolls initiative, determining the order of combatants’ turns.
- Take turns. Each participant in the battle takes a turn in initiative order.
- Begin the next round. When everyone involved in the combat has had a turn, the round ends. Repeat step 4 until the fighting stops.
Note that during surprise:
Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends.
In this case, the order of events is:
Combat positions are set up / drawn up / etc. (or have already been done)
Everyone rolls initiative
Players each take a single turn. There are a couple of ways to DM this - I couldn't find a RAW interpretation on exactly what order this occurs, since the Wizard may not have the highest initiative but is taking the first action. RAW says initiative, but the DM may want to consider manipulating this so the wizard doesn't get an extra round
Combat enters the normal initiative, and proceeds accordingly. Orcs are slain, treasure is looted, and celebratory debauchery ensues.
