Ambush was added in Beta 42.0 as a replacement for Final Charge during the Bravery rework. It kept the same sprite.
For 6G, you can summon any monster from your hand with a cost of 10G or lower. Afterwards, an effect that works the same as Receptionist 3's is triggered, summoning a random monster from the enemy's hand onto the enemy's board.
If both monsters are summoned, yours will then attack the enemy's monster right away. If the enemy couldn't summon one, yours won't attack anything.
Combos[]
Ambush is highly effective on monsters that are either high cost, or can minimize the damage taken from the trade. For example, Everyman completely negates any damage from the attack, with Sad Dragon being a close contender thanks to their Dodge. This being said, perhaps the most powerful Ambush combo is using Ambush on Mettaton NEO, essentially acting as a 6G board wipe, making it the one of most cost-effective board clears in Undercards, if not the most.