What is the most severe engine failure for modern turbofan?
Here are my initial finding so far, and I will add information that I find along the route:
There seems to be two types of engine failures. Tested and untested ones. The ones that the engine are certified for through test are the ones that are likely to happen in the fleet life. And therefore, the engine manufacturer must show that those kinds of failure will not put the life of the passenger at risk. The other are unexpected incident that shouldn't have happened and are certified through numerical analysis.
The most extreme scenario in the tested failure category seems to be the fan blade out. The unbalance created by losing a fan blade creates huge vibrations and breaking torque when the fan rubs against the casing. In addition to that, the compressor is stalled so you have backward flow in the engine and huge ball of fire coming out. (see this video). 
                
            
To obtain the engine certification, the manufacturer must demonstrate during a dedicated test that all broken blades remain in the engine and that no heavy parts will exit either in direction of the aircraft or fall to the ground.
Sadly, it doesn’t work as expected and recently two CFM56-7B have had FBO event that didn’t end as well as planned.
Southwest Airlines Flight 3472 and 1380.

Now the untested failure options. Two different examples came during my search, rotor disc rupture and engine loss. Both should never happen but cannot be mitigated if they happen and that's why they cannot be certified.
Let’s start with the American Airlines Flight 191, which engine number one was lost during takeoff. Whereas it is not the engine that failed but the fixations, the consequences where tragic.
Secondly the rotor disk rupture. This time it is not the blades (fan turbine or compressor) that fell off but the whole disk retaining them that fails. This seems mostly due to manufacturing defect with crack propagation not detected during maintenance. The kinetic energy of those disk is such that it would require a tank shield to prevent them from escaping from the engine which weight would be horrendous. Sadly, they are more example of those:
Delta Air Lines Flight 1288 (MD-88, the NTSB determined the most probable cause of the accident was a fracture in the left engine's front compressor fan hub)

National Airlines Flight 27 and United Airlines Flight 232 (DC-10 Tail engine fan disk rupture leading to flight control losses, loss of cabin pressure and passenger sucked out)

Qantas Flight 32 (A380, turbine disk disintegrated, causing extensive damage to the nacelle, wing, fuel system, landing gear, flight controls, engine controls, and a fire in a fuel tank that self-extinguished)

Air France 88 (A380, Fan disk failure. The examination determined that the hub failure was caused by a low cycle fatigue cracking process which originated in the part’s subsurface. 21 month later a 150kg piece of the engine was located 4 meters below the snow and was recovered in Greenland.)