I think I'm missing something in how treachery is used in Sea of Blood. Per the SoB rules (p. 33), the overlord may "increase" his treachery by spending XP to purchase points of a given type up to the limits shown on the respective avatar sheet. It costs between 15 - 20 XP to purchase 1 point of treachery. Given the cost per point, it doesn't seem worth bothering with treachery at all.

In contrast, dungeons in Well of Darkness provide the overlord with 10 - 12 treachery points. Does the overlord start with any treachery in SoB or must it all be purchased? The use of the word, "increase," led me to wonder whether the overlord started with a set amount of treachery but I can't seem to find any reference to that in the rules.

Any help on this issue is appreciated.

Marximus said:

The OL starts with 0/0/0 Treachery per dungeon. That is the set amount of treachery he starts with. From 0-1 is still an increase and does not in any way imply a non-zero starting point.
He can increase that 1 point at a time by purchasing more treachery through the campaign.

Each point purchased increases his Treachery per dungeon by one, so eventually, if he purchases 4x10, 3x15, 2x20 treachery he will end up with some variant of 4/3/2 Treachery per dungeon to use.

A limited amount of treachery is very powerful in Advanced Campaigns.
In RtL, with Treachery also used in Lt encounters, a large amount of Treachery is devastating. In SOB it is much less important, though an early event Treachery for Crushing Blow is practically mandatory.