Infinitesimal is an achievement earned for defeating the Grand Magistrix Elisande encounter in the Nighthold instance on Normal difficulty or higher, after carrying out a series of steps involving the Infinite Whelpling pet.
Specifically, the pet must be active and be moved through two overlapping bubbles left behind by each type of boss's elemental adds, and kept there for several seconds. After doing this, the whelpling will be transformed into a hostile drake that must be slain. Doing that, and then defeating Elisande, will award the achievement.
- Patch 7.0.3 (2016-07-19): Added.