History of Conservative Banker

Conservative Banker is a Gambit Triumph: as a team, win a match in which you summoned only medium Blockers.

How to complete

1. Blocker size depends on motes banked at once: roughly 5+ motes summons a small Goblin, 10+ a Taken Phalanx (medium), and 15 a large Knight (Destinypedia, which may be dated).
2. Bank in the medium range (10-14 motes) each time and never in the small or large range, then win the match.
3. A Bungie forum thread from 2018 reports that banking exactly 10 motes sometimes did not count as medium and that 11 counted. The triumph was also reported as bugged then, so bank 11-14 to be safe. This may be outdated.

No reward is confirmed in the sources I opened.