Start with the Roman sword.  2/3 from the base of the blade to the tip, drill a hole.  Weld a chain into the hole 1/2 length of the blade of the sword.  At the end of the chain, weld a tungsten ball with spikes on it.

Many things you can do with this close up, and working around a corner or a shield.

Makes in one hand a flail and a sword.

After reading a few comments (thank you) I have further ideas:

In the chain, incorporate many fish-hooks that would ensnare flesh if the chain were to drag against the enemy. Also, these hooks could help the chain grab onto the enemy sword.  We are only talking about a 4 ounce chain and a four ounce tungsten ball; yes it would slow the sword, but the speed of the ball and chain would be very fast.  I think this could be tried out with wooden mock-ups, weighted to be realistic.

Another approach would be to start over. Saw off the tip of the sword (1/4 of the blade) and reconnect it with a chain 5/8th  the length of the original sword.  Use the fishhooks on the chain AND the flat sides of the reattached portion of the sword.

I think such a weapon would look fearsome, but the effectiveness of each incorporated weapon would be decreased by combining them.

If you were trying to use the weapon like a sword, you would have an extra weight (possibly an extra weight in motion) attached to your blade.  This would interfere with any movements and control of the sword.  Swords are usually balanced with a center of mass close to the grip.  A weight attached to the blade would have the opposite effect.

And if you were trying to use it as a flail, there would be a blade in the way of the chain.  If you wanted to rotor the spike ball (as one would a sling) to build momentum, it would just wrap around the blade.

What if the chain stowed securely in a slot down the middle of the blade so that the ball sat rigidly on top of the blade Until you press a button and release it?  It would wield more like an axe or mace in rigid mode... I’m not sure the sword blade really adds much still because the spikes do all the work, but maybe you could do some wicked chopping if the ball doesn’t get in the way too much. With the chain loose though... I think the sword blade is more of a liability. If the goal is to hit around shields, the blade could lodge in the shield and get stuck.

I don't think you would be able to use the sword part of the weapon very effectively.

Part of the reason swords were effective is because the were agile in both attack and defence. Adding a ball and chain to the end would completely remove this element.

Additionally I'm not certain that the sword blade would be able to withstand the torque applied by the flail element and would bend or break (or at best be really floppy and difficult to manoeuvre). Swords are designed to flex and be springy so they don't snap but this is not the kind of force they are designed to take.

I saw an interesting video from Lindybeige around flails and how they probably weren't actually used much at all in historical settings due to difficulty of control and safe usage (IIRC).

https://www.youtube.com/watch?v=O-y6oirEsZA

https://www.youtube.com/watch?v=RjzE8YMkC5s

Perhaps a rather thick, rigid blade would work better in this case? Katanas, for instance, are designed to be very thick, and rigid - They apparently sooner crack rather than bend. Maybe that could work in the design's favor.

The hooks in the chain would become trapped in the flesh, clothes, armor or shield of your opponent and while your sword is bound, he kills you.

Which is why flails didn't have them. 

Japanese lawmen, back when everybody carried swords, used to use long poles with hooks and chains with hooks attached to pin down drunken sword fighters.  Two or more would hook the clothing of the drunken sword fighter while a couple more went in and shackled him.  No one dead or seriously injured.