My mind immediately jumps to weird theories. Its more likely that its something simple like the ones mentioned above where he jumped in when someone of higher status didn't take the role (death or chickening out). However, some other things I thought of that could fit the line.
1. Honour made a deal with Odium to create the Oathpact and trap Odium, but used a loophole. The deal would be similar to Tanavast's suggestion of a contest of Champions. Odium expected 9 Heralds but Honour added Taln as an extra one that was allowed by the specific wording of the deal but wasn't something Odium expected. It could explain why there are 9 unmade (They would be Odium's champions).
2. Taln was a powerful member of Odium's followers that turned.
3. The Oathpact was created without Taln. Taln went to the Nightwatcher (or Cultivation) asking his boon be that he's made part of the pact. It seems like the perfect thing for Cultivation to grant (if she has the power) since being part of the pact is really not just a boon but also a curse. It's possible that he didn't even ask to be let into the Pact, but if he did love Ash then he merely asked for the ability to protect Ash, and Cultivation granted it this way.
