Talk:Harvester/@comment-80.200.44.151-20131221141346/@comment-84.101.135.186-20131225103603

Stalker has 5% chance of appearing after you kill a boss. Dice rolled in every mission until he appears. After that I believe you have to kill another boss to get him to appear again.

Note that the Stalker doesn't remember which bosses you killed since he last visited you, only the bosses you visited at least once.

I think it's safe to assume that Zanuka works just about the same, but with grineer invasions instead of bosses.