This is a Midringed Blue-Ring, only found in Lembeh, Sulawesi. They spend most of their lives under rubble, so are very rare to spot.
This one we found at around 20m in a heavy current at Critter Hunt, one of the dive sites here in Lembeh.
Though renowned for being extremely venomous, they are not really that dangerous to divers. If you really really harassed one (which we never do) and got bitten, then as lng as someone ventilates you for about 30 minutes, you will be fine….