Dark Electrum

 is an uncommon catalyst which can be used in the process of upgrading Adaptive Augments related to the Metal Part I episode.

Obtained from

 * Metal Gotham City daily and weekly missions
 * Metal Pt. I: Scarlet Speedster
 * Metal Pt. I: Batcave Breach
 * Metal Pt. I: False Idols
 * Metal Pt. I: Monsters of Metal

Trivia

 * Three pieces of Dark Electrums are needed to breakthrough milestone rank 15 of an augment.