XCOM: Enemy Within Developer Blog: The Seeker

Firaxis' lead designer Ananda Gupta has penned a developer blog post to describe one of the new enemies added by the XCOM: Enemy Within expansion to XCOM: Enemy Unknown, the stealth-focused Seeker. Here's a snip:
Making the Seeker a flying unit made a ton of sense; an assassin has to have a lot of mobility, especially if we want it to make hit and run attacks with a close-quarters ability. Stealth would also help it approach the player's squad without getting chewed up before it can get to grips. But stealth proved very difficult to get right; in a turn based game, there are defined windows for player action, and we needed to make sure that players didn't spin their wheels hunting the last cloaked enemy on the map. The Seeker's stealth was initially unlimited, but this was quickly changed to a fuel-like system both for multiplayer balance reasons and so that (hunt the Seeker) didn't become a problem during abduction and terror maps once it was clear the player had a strong advantage.

Seekers appear in pairs; this is pretty tightly controlled. Originally, Seekers could appear in moderate numbers, but one too many missions where the entire XCOM squad was simultaneously strangled led us to change that. To compensate for their small numbers, they will often defer combat until XCOM encounters another alien group; this keeps the player suspicious, and offers a choice: try to locate and destroy the Seekers immediately, or move on and hope you can deal with them when they combine with other aliens on the map?

There are several counters to Seeker stealth, but they fall into two categories: detecting the cloaked Seeker, since Seekers cannot Strangle if they are visible, and defending specifically against the Strangle attack. In the former category, Bioelectric Skin (a gene mod that is unlocked from the Chryssalid Autopsy) and Snipers' Battle Scanners will detect Seekers in their radii of effect. For the latter type of defense, the Respirator Implant prevents strangulation as well as granting some bonus health. And many advanced armors confer immunity too, due to their life support systems and heavy construction.