Eximus

Leaders are special 'elite' variants of standard enemies introduced in Update 12 that can appear in any mission type. They come with special abilities, some of which are similar to Tenno and are sometimes much stronger than an average member of their faction, though effects vary from type to type. Leaders spawn 10-15 levels higher than the level average for the map, and can pose a reasonable challenge for under-equipped or inexperienced Tenno.

Almost any base unit type (Osprey, Lancer, MOA, etc.) can spawn as a Leader type. They can easily be identified by their special auras and larger sizes, and frequently have unique textures/skins to make them more visually distinctive.

Corrupted Ancients in the Orokin Void use the word 'Elder' in place of 'Leader' for unit names, and may use different prefixes (Sanguine instead of Lifesteal, etc.). Aside from terminology, however, their Aura effects are largely the same.

Upon death, leaders provide four times the regular affinity gain of their regular counterparts, added after their level differences.

Blast
"Defends itself with a radial Blast attack."

Blast Leaders are capable of casting Seismic Shockwave, which knocks down nearby opponents. They are not noticeably more durable or dangerous than their base type, aside from any benefits gained from increased level.

Energy Leech
"A tank who drains Energy from nearby enemies."

Drains energy from nearby enemies at the rate of 5/s.

Energy Leeches are common amongst the Infested.

Fire
"Heat based attacks and Auras"

Leaders with the ability to create Fire Blast similar to the one used by General Sargas Ruk.

There are no Fire Elders for the Infested - instead, they have Caustic Elders; these behave largely the same, and even use the Fire Blast-type attack.

Guardian
"Restores shields of nearby allies."

Guardian Leaders are especially durable, gaining a significant boost to their own shield durability as well. They release radial shield-restoring pulses, similar to the healing wave effect of Ancient Healers.

The Infested are unlikely to ever spawn as Guardian Elders, as the Infested possess no Shields outside of very specific, usually Tenno-engineered circumstances.

Ice
"Cold based attacks and auras."

Cold Leaders can generate a temporary Snow Globe-esque barrier to protect themselves and allies. Their version follows them when they move as opposed to being stationary like the conventional Snow Globe power, and has a unique and distinctive Truncated Icosahedron design to it. They have additional ice-based attacks as well.

Ice Leaders are among the most dangerous variety, as their ice slowdown aura is fairly large, whilst their snowglobe protects them from ranged attacks. Certain targeted powers, like Oberon's Smite and Volt's Shock, can damage the target through the snowglobe. Like the regular Snow Globe, inflicting enough damage will cause the Ice Leader's Snow Globe to disintegrate, leaving them vulnerable.

Lifesteal
"Life leech weapon, Enhanced health."

The prefix and subtitle for this leader type are slightly misleading; these enemies do not seem to steal the target's health per se. Instead, they regenerate health by inflicting damage. This is easily observed after damaging them, as they will regain health from damaging shields and defensive powers such as Iron Skin.

Infested Leaders and Elders use 'Sanguine' in place of Lifesteal, but otherwise behave identically.

Mag
Attacks are Magnetic, same effect as the Infested Ancient Disruptor's special attack, draining enemies of shields, power, and disrupting the screen.

Corpus Crewman variant seen with sub-text; "Electric based attacks and auras". Generated a constant magnetic aura that caused HUD disruption, shield loss, and energy loss.

Venomous
"Toxic attacks and auras."

Toxic attacks and aura.