XCOM: Enemy Within will introduce new extraterrestrials for players to battle. The most frightening of these new additions seems to be the Seeker.
"One of the things we wanted to do in Enemy Within was to get the player looking over her shoulder more often," explains lead designer Ananda Gupta on 2K Games' blog. "We have terror maps that can get out of hand with zombie infestation, but the aliens were missing a subtle assassin, something that can attack suddenly and without warning when the player least expects it. The Seeker was an obvious candidate. It never made it past the early concept stage on Enemy Unknown, when we wanted to emphasize the 1993 XCOM’s enemy lineup."
Seekers are squid-like robots can fly across the battlefield. They use a cloaking device in order to flank the enemy or pick off isolated units. Their main weapon is a plasma blasts. However, if they're close enough to an enemy while cloaked, they can use a Strangle ability. This allows them to latch onto a target and paralyze them while they drain their health with each turn. If the Seeker is dislodged from their target or killed, their prey still has to waste a turn recovering.
These robots only spawn in pairs (if that) so they're not going to defeat your squad on their own. Instead, they wait until you've engaged other enemies and then make their move. Keeping the Seekers off your squad will be a lot harder when you're also trying to keep a group of Sectoids at bay. Some players may opt to hunt down the Seekers first to avoid such a scenario but that's not an easy task in a hostile area.
The good news is that a Seeker can't use their cloak indefinitely. Instead, it drains fuels over time so they won't be permanently invisible. Furthermore, players can detect it with the Bioelectric Skin genetic mod or Snipers' Battle Scanners. XCOM agents can also be immunized from strangulation if they're outfitted with Respirator Implants or certain heavy armors. Presumably that includes the new Mechanized Exoskeleton Cybersuits.
Enemy Within is the first expansion pack to XCOM: Enemy Unknown. In addition to new equipment and enemies, it will also introduce new upgrades and abilities. Players can also look forward to new maps to test their tactics. The expansion will be released on PC and Mac in November. 2K Games will also release a standalone package called XCOM: Enemy Within - Commander Edition on Xbox 360 and PS3.