These are the values that should be in your unittype to make sure the champion gets spawned.
<Allegiance>Kingdom</Allegiance>
<Unique>1</Unique>
<IsNPC>1</IsNPC>
<NPCSpawnLikelihood>200</NPCSpawnLikelihood>
<IsChampion>1</IsChampion>
<SpawnRating>1</SpawnRating>
Allegiance - Kingdom or Empire
Unique - can it spawn once or more than once?
Likelihood - the weight (chance) for this unit to appear instead of others.
IsChampion - puts it in the champion's list to be seeded
Spawnrating - goes from 1 to 5, 1 being champions you can find just next to your starting position, 5 being strongest. Spawnrating also sets what level they are (1, 3, 5, 7, 9 respectively). For that reason it's a good idea to base your champion off the ones found in CoreUnits.xml so that they are properly balanced with traits and equipment.
The core game has 10 champions for each allegiance for each spawnrating, so keep that in mind as you assign likelihood.