99) Comes a horsemen

The first episode where Kronos made an appereance. And already he isn't a stranger to Duncan McLeod. The highlander confronted him once in the Wild West, where Kronos was known as Melvin Koren and left a trail of death and violence wherever her went.
Duncan's part-time lover Cassandra reveals then his true identity and the even more shocking fact that Methos and two other immortals rode with him during the bronze age calling themselves the Four Horsemen.

After Kronos stabbed her and she turned out to be an immortal, Methos made her his personal slave. Still Duncan gives Methos a chance to escape and he returns to Kronos who already paid him a visit earlier. He wants Methos to kill the Highlander for him, but things go a different way, when Cassandra confronts Kronos.

Duncan follows her and after she gets knocked out by Methos, it is Kronos and Methos who fight. That is until Methos interferes by setting the battle ground on fire and the flames seperate the fighters. Although Kronos is quite upset about it, the hint that Methos knows where the other two original horesmen are, is too tempting ...

100) Revelation 6:8

While Kronos and Methos meet with Silas, the first of the other two horsemen, deep in the forests somewhere in East Europe, Cassandra and Duncan use the ressources of the Watchers to try and trackdown the others.

However they come to late to a mental institution where Caspian, the last of the horsemen, has been imprisoned, but set free shortly before Duncan and Cassandra arrive. But a hint left by Methos sents them to France, where in an abandoned submarine base, Kronos presents his true plan.

He created a deadly virus and plans on using it to either destroy the world or force it into submission. In the meantime Methos sets up a meeting with Duncan on holy ground. The highlander is still not sure what to think about his friend.

It seems though that Methos really cared for Cassandra, but as Kronos was becoming jealous about them getting along rather fine, he claimed her and Methos was too afraid to interfere. But he tells Duncan that after Kronos raped her, he saw her fleeing the campsite and that he could have stopped her but didn't.

Kronos uses this meeting to kidnap Cassandra from her hotelroom. Stabbing her with the very same dagger she used all those years ago on him to get away. But even sending Caspian and Silas out to get Duncan doesn't stop him, he kills one and finds the secret base.

There Methos confronts Silas to stop him from killing the unarmed Cassandra, while Kronos and Duncan have a duel of their own. Kronos and Silas both lose their head and their energy both go over to Methos, who is nearly decapitated by an upset Cassandra.

119) Not to be

The second of a two parter, Duncan is in a parallel world, where he never existed. One of the results of this is that Horton, a crazy watcher, was never stopped and still hunts immortals. While in the other universe Methos is quite content, in this universe only his fiance keeps him steady.

She is however a watcher and naive enough to believe that if Horton would only meet him, he would see that not all immortals are evil. But things go out of hand. Horton and his killers attack and try to kill Methos, who is saved by Kronos.

Both join forces and even recruit Ricky, they make a lot of money and finally decide to get rid of Horton (who got away) and the watchers. Kidnapping Joe Dawson and beheading Ricky, upsets Duncan so much, that he intervenes by beheading Methos. Which brings him back into the original verse, where he saves the day with help of the Methos there.

Archangel & Armageddon

In those two episodes we have a re-apperance of Kronos, at least kind of. It isn't Kronos himself but a demon called Ahriman, who uses the appearance of Duncan's worst enemies, to get the better of the highlander.