
Some rough math on Tristan's reaction ability, ignoring bag-manipulation/Favor/Covenant/etc effects:
With 10 total bless/curse remaining in bag during the current test, there's about a 5% chance of triggering him.
With 15 total, there's about a 10% chance.
With all 20 in the bag, there's about a 15% chance.
(For a base 16-17 token bag.)
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(Personally, I've only ever seen him get played full-price, but even then the effect is quite strong and flexible, and is an especially reasonable choice for small-collection investigators looking for a generically-useful ally.)