Sneak Build for Cheesing Black Sacraments and Heists - Lame noire Construire PvE - ESO

• Subclassing
• Gear
• Active skills
• Extra passives

This build is designed for max movement speed while sneaking and keeping up Shadowy Disguise. Do not use this in combat (dungeons, etc.) as you will do no damage.

This build is over-provisioned in terms of speed, max magicka, magicka regen and minimal detection radius, meaning you're able to replace some skills/gear pieces without breaking the build. 

Subclassing

The only class skill line that's essential is Shadow for Shadowy Disguise. Draconic Power is used for its healing and health recovery, which is used to offset the cost of Spell Symmetry. Dark Magic is slotted for its cost reduction passive, but can be considered optional. Use one of your class skill lines in this slot if your character isn't a Nightblade, Dragonknight or Sorcerer.

Gear

• Vesture of Darloc Brae reduces the detection radius and gives a massive sustain boost.
• Night Terror also reduces the detection radius. This build only needs the 3-piece bonus.
• Ring of the Wild Hunt gives a massive speed boost.
• The single piece of Magma Incarnate offers double recovery which is nice, but can be easily replaced with another 1-piece bonus (e.g. only magicka recovery, max magicka).
• The type of weapon doesn't really matter, but this build uses the bow for the small speed boost after a roll dodge (useful if you have to escape combat, otherwise not necessary).
• The body traits/enchants are all Infused/Max Magicka for Shadowy Disguise uptime. 
• The Magicka Recovery glyphs on jewelry also help with magicka sustain (when recovery kicks in when standing still). Reduce Spell Cost glyphs don't work on Shadowy Disguise as of Update 46. The Infused trait boosts recovery, Swift boosts speed. Run more Swift if you're missing other speed buffs.
• The Trainee pieces are used to fill in the gaps and for some extra resources. These can be replaced with two pieces of Druid's Braid or any other two pieces with a useful bonus.
• The weapon trait/enchant doesn't matter, I recommend slotting a poison for the Dark Brotherhood daily quest (e.g. random trash poison or the ones from the daily rewards). Powered is the only trait that can be useful outside combat.

Active skills

• Shadowy Disguise is Shadowy Disguise.
• Elusive Mist can be used as a movement/escape tool. Keep in mind that it's a Criminal Act.
• Spell Symmetry is used to exchange health for magicka for infinite magicka sustain as long as your health recovery is running. Using this skill does not break sneak! Keep in mind that higher vampire stages reduce or halt your natural health recovery.
• Inner Light is slotted for its Max Magicka buff and Mages Guild passives.
• Coagulating Blood is a heal which also provides extra health recovery as a heal over time. Using this skill does not break sneak! Keep the recovery running while using Spell Symmetry to keep up Shadowy Diguise. Keep in mind that you're losing this heal over time if you are Vampire Stage 4. Use a generic heal (e.g. Resolving Vigor) if you don't have Draconic Power. 
• Barrier is slotted for the Magicka Aid passive in Support. You can also slot the Shadow or Draconic Power ultimate here and for some extra class passive buffs.

Extra passives

• Shadow passives help with recovery.
• Draconic Power's second and third passive help with health recovery.
• Dark Magic's Unholy Knowledge passive helps with sustain.
• Light and Medium Armor passives help with sustain/recovery and movement speed.
• Bow's Hasty Retreat gives a little speed boost after a roll dodge. 
• Vampire is useful for the Dark Stalker passive, specifically the ‘Ignore the Movement Speed penalty of Sneak’ part. I do not recommend using high Vampire Stages for the other passives: you don't need them for this build, but you do need health recovery.
• Race doesn't really matter, Khajiit and Wood Elf have useful sneaking/thieving passives.
• Legerdemain's Improved Hiding passive helps with sustain.
• Dark Brotherhood and Thieves Guild add some flavor.