Currently, the World Guide version and the Chasmfiend Bonus Content variant both point to the same entry, and are labeled as Tier 3 Boss.
However, according to Chasmfiend Vol. 1, the Chasmfiend Bonus Content variant should have different stats and should be a Tier 2 Rivel.
Following the approach used in Advanced Adversaries, it should probably be marked as Rivel Variant and have its own separate entry.