Last night I decided to dust off Skyrim and finally play through as a Mage. So far it is fun. But I have been using a bow for sneak attacks at the start of each fight.
The problem being, most Destruction spells I have used are either (a) not long-range enough for sneak attacks, or (b) WAY too visible/noisy.
Do Mages have a spell that works well for sneak attacks?
According to a post in this thread, magic attacks are all noticeable, but don't get a sneak bonus anyways.
Casting spells make noise and they don't got a sneak bonus anyways.
There is a spell to allow quiet casting, as mentioned in a different post:
You can sneak+cast with Quiet Casting, yes. However, doing so, with most spells gives away your position almost immediately. And there is no damage bonus for sneak attack spells.
However, as also noted in the post, this will most likely give away your position and defeat the entire purpose of sneaking to begin with.
The Illusion school allows you to one-shot, at range, opponents who are in a group.
Let that sink in.
You cast a spell and sit back while his friends take him out. It's not instantaneous, but it's very sneaky.
If you want to play as a stealthy mage, you need to take special care to keep your Illusion skill level & perk purchases on par with the level of your opponents. See the 2nd half of this Answer for a guide.
Open by casting Fury/Frenzy/Mayhem (Dual cast if required) into a group of opponents. Their reaction? "Hey, I saw a spell flash over there, but there's a madman right here trying to kill me!" Take advantage of the the distraction to relocate in case they come searching.
Repeat as required until you're left with a single opponent. Feel free to take him down with the Destruction/Conjuration/Restoration spells of your choice. The Impact perk can be very useful for controlling a single strong opponent and, past the novice level, destruction spells work well at range. Yeh, bows or better crossbows make a decent 1st shot on your last opponent, but after a while your bow skill levels should fall behind your destruction skill levels and the bow will become less attractive.
Spell sounds don't carry too far. If you're using Lydia as your active follower, they're more likely to hear the sounds from her heavy armor than your spells. So the Quiet Casting perk is most useful when casting a spell around the corner from a foe or near Falmer. This might be most useful when casting Muffle, or an AoE Fury effect in tight quarters. Invisibility might be helpful to re-acquire your stealth status.
The main disadvantage of this approach is that these spells are limited by the level of your opponent and your opponents will, generally, level with you. As such, purchase the following perks before the relevant milestones:
Let's be honest, these spells are for picking off the low hanging fruit of groups, so you are unlikely to want to target anything over 45. Seriously, stop asking Tolfdir for trouble.
Draugrs can reach level 80/81 which makes most illusion spells unable to work on them. Though the actual level might be even higher but that is the level where I start noticing DC Frenzy failing and DC Pacify failing on more targets.
Master of the Mind perk @ Skill 90. After that, of the common Draugr, the worst is the Level 45 Draugr Death Overlord. Leaving only Named Draugr & Dragon Priests as able to level with you, and those lone survivors of "The Illusion Games" need to be handled with other methods anyway. Given that very few critters scale that high, the game gets increasingly easier after 50 anyway.
If you do not mind not being a pure mage - under Conjuration school there is Bound Bow and if you have the Dragonborn DLC, Bound dagger. These spells will affect and are affected by Conjuration and Archery/One Handed. They also receive sneak attack bonus as physical weapons would which makes good choice of spell for mages to initiate an attack.
Using the weapon itself will produce no more noise than a physical weapon. So even without Quiet Casting, you can summon the weapon from a distance and close in for the attack - just make sure to note the summon duration of the weapon, if you take too long the weapon might unbound.
If you wish to purely rely on the five schools of magic - spells like Firebolt and Ice Spike can be used over long distances, but to practically fire it without Quiet Casting and without being heard, you will need to cast from distances where the target is barely visible. So you will need to invest in Illusion school for the silent cast and you have to cast from behind/ out of the line of sight of your enemies.
Note that while the perk will hide the noise from your spell, most spells produce light which will increase detection from your enemies so you need to cast from outside their line of sight to ensure they do not spot you visually.
Tactically, you can use the Magelight spell from the Alteration school while hiding in cover to draw you enemies' attention away from you then cast a spell from behind them followed by Muffle and/or Invisibility to sneak away before repeating the process. Important that you DO NOT cast the Magelight within the line of sight of your enemies, the increase the light around you at the point of casting will almost immediately give away your sneaking attempt. Do so from cover so that they will investigate the light and back-face you instead.
The runes spells from Destruction school can also be used for sneak attacks, you have to watch the movement route of your enemies and plant it along that route, retreating away before they return to step on it while patrolling. Alternatively you can plant a rune near a pillar/wall and lure the enemy to it using Magelight from cover This tactic will greatly reduce the chance of being detect since you can almost be fighting while physically be behind cover.
Another viable tactic is to use Paralyse, though this can only be done with Quiet Casting since you have to be relatively close - definitely within audible range, to cast paralyse. But if you do it with silent cast and from behind the target, the target will be unable to move for a few seconds, granting you enough time to blast another few spells and retreat away before the target can move to find/chase you.
Be careful to watch for the direction the targets fall while you do this, sometimes it might fall into a position where it can visually see you. That will enable it to detect you while you try to blast it with spells due to the proximity - you are already quite near to it to cast Paralyse and light from your spells.
While this is not entirely stealth, the summons under Conjuration school and the Frenzy series of spells under the Illusion school can make the opponent distracted while you blast them from afar. With sufficient proficiency in Sneak, there is a chance that you actually remain undetected while you hit them with your spells.
If bound weapons are used there are certain(?) conditions that might trigger the sneak attack bonus even while you opponent are engaged in battle - though I have yet to confirm how this works but I have successfully triggered sneak attack bonus on several occasion using this method.
This method might potentially kill the target fastest but will require a high proficiency in Illusion. The Calm series of spells includes Calm, Pacify, and Harmony. While Harmony will be more effective and useful in most situations, it requires a longer casting time plus more magicka. When the level of the enemies get higher you might need instead to use Pacify with Dual Casting to calm the target.
Practically, this method is rather simple - hit the target with the most painful spell you can cast, calm it down, repeat. A calm target even if it detects you will not attack, therefore you can just casually move into position and cast another spell to hit again. If you are using bound/physical weapons to damage it will also be very easy to move out of the enemies detection to hit it with sneak bonus.
If you don't mind using Mods, there are several Mods which add Magic Sneak Attacks, for example Spell Sneak Attacks 2 .You can get those Mods either using the Steam Workshop or on Nexus Mods.
I am playing a stealth destruction mage. I got my Sneak up to 38 simply by always sneaking in Dwemer ruins and during the College of Winterhold quests. I haven't done much other combat throughout the rest of the game. Once my Sneak passed level 25, I noticed when attacking even with the loudest Destruction spells like Chain Lightning and Fireball, it was rare that Falmer and Dwemer constructs would fully detect me before I killed them and, combined with Impact, they rarely had a chance to even hit me back even if they did eventually detect me.
Once I got the Quiet Casting perk from the Illusion skill tree in combination with the 38 Sneak, my ability to cast stealth destruction attacks just became ridiculous, borderline game breaking. I was able to sneak into the gated room of skeletons with a dragon skeleton boss in The Staff of Magnus quest and Chain Lightning all the skeletons, equip Flame Cloak, and Fireball the dragon, all without a single enemy ever detecting me. They all died not knowing what was going on and never rising above a quarter of "open eye" detection, even when I was directly attacking them. Including some of the skeletal mages that got about 5 feet away from me during their partial detection.
So, in summary: can you play a stealth destruction mage without invisibility or muffle and cast spells without being detected by creatures, constructs, or undead? Absolutely. Get your Sneak to 38 and get Quiet Casting. You'll be able to wipe out an entire room of enemies using loud AoE Adept level destruction spells without a single enemy detecting you.
