Gear Review
Balorgh is quite a powerful monster set to use on the Magicka Nightblade bomber as it gives additional Spell and Weapon Damage along with additional Penetration based on how much Ultimate you spend. The more Ultimate points you have stored (up to 500), the stronger your bomb gets.
Vicious Death is the primary mechanism to which you kill people en masse. Once someone dies to you, their death can set off a chain reaction in the group you're targeting after your bomb and skills go off resulting in more deaths. The more people die, the more explosions there are resulting in even more deaths.
Mechanical Acuity offers guaranteed Critical Damage once it procs on your Proximity Detonation or Soul Tether or any direct damage you deal. While your Shadowy Disguise also offers a guaranteed Critical Strike out of stealth, the Mechanical Acuity extends the window of time you need to keep apply Critical Damage to enemy players.
You ideally want to have Mechanical Acuity on one bar and not active on both bars as you can control the proc much more easily.
The two piece bonus of Seducer or other sustain options is good to have on your backbar. A large part of your time as a bomber is spent almost constantly using Shadowy Disguise until the timing is correct or if the coast is clear for you to position yourself for a bomb.
Weapon Types
You don't necessarily have to equip dual wield Swords to be an effective Magicka Nightblade bomber. You can also use Greatswords as they have the same effect as dual wield Swords along with the same amount of Penetration offered by the Sharpened trait.
If you're a Vampire using Simmering Frenzy to boost your Spell Damage at the cost of your Health, you also have the option of dual wield Axes or two handed Battle Axe to boost your Critical Damage.
Skills Review
Sap Essence will be your primary Area-of-Effect damage skill once your Soul Tether/Proximity Detonation combo goes off in the group of enemy players. It acts as not only a damage ability but also a heal for yourself to prolong the effects of your bomb.
Concealed Weapon is not only good for burst damaging a single enemy should you need to kill or gank them fast but it passively increases your movement in stealth which is great when you're using Shadowy Disguise. This is also a flex slot for Lotus Fan, a gap closer that applies Minor Vulnerability should you have issues getting close to your targets.
Shadowy Disguise is your main method of invisibility and stealth. Crouching everywhere all the time can be very slow especially if you're not a Vampire.
Camouflaged Hunter is pretty much guaranteed to proc as you're bombing since almost all the time someone's flank is exposed to your attacks. It gives Minor Berserk should you not have the time to pre-buff Combat Prayer before bombing.
Inner Light is a very nice passive ability to have as it increases your Max Magicka while guaranteeing Spell Crit from Major Prophecy especially if you're using Immovability potions.
Soul Tether is your primary Ultimate you need to use and try to perfectly time with the explosion of your Proximity Detonation. Eye of the Storm can be an alternative Ultimate depending on the situation.
On the backbar, you have Siphoning Attacks to help your sustain as it's very nice to pop it halfway through its duration or refresh it after the duration for much needed Magicka. Otherwise you can use the Psijic skill Deep Thoughts as a sustain option instead of Siphoning Attacks.
Proximity Detonation is of course a must as it's a strong AoE ability that scales off of how many targets you're hitting.
Combat Prayer is optional and can be replaced by Radiating Regen but it's quite a strong burst heal in situations where you may be pulled out of stealth or perhaps you're bombing with a friend and they need a heal!
Shadow Image is also optional and can be replaced with something of your own preference, if you'd like to successfully bomb a group then bomb the people resurrecting them again, you need an easy way out and teleporting back to your Shadow Image to live to bomb again isn't shameful at all.
Channeled Acceleration not only gives a much needed speed boost from Major Expedition but also grants Minor Force which is an additional Critical Damage modifier that can help your bomb damage.
The backbar Ultimate listed on this build page is Temporal Guard and it's not exactly meant to be used as you want to build up your Ultimate for a Soul Tether. In the worst case scenario where you don't want to die you could use it to get away or simply use it passively for the Psijic Shield from the Concentrated Barrier passive in addition to the Minor Protection you gain from slotting it.
Other Ultimate options can include Incapacitating Strike or the Restoration Staff Ultimate.
The order of your prebuffs go something like this: Channeled Acceleration (in stealth), cast Proximity Detonation and then cast Combat Prayer, Shadowy Disguise and time the bomb.
Craft
Warfare
Fitness
Champion Points Review
To see what each Champion Point does, hover over or tap each one shown in the allocation chart.
Fighting Finesse, Backstabber, Biting Aura, Master-at-Arms are the primary Warfare Slottables you will need. Your Nightblade bomber's toolkit in terms of passives and skills for bombing mesh incredibly well with the added damage modifiers from Fighting Finesse and Backstabber. Your primary damage skills are both direct damage and Area-of-Effect based so Biting Aura and Master-at-Arms will be essential to maximize your damage output.
Bastion, Rejuvenation, Ironclad, Boundless Vitality are the primary Fitness Slottables you will want. Bastion is particularly strong as it not only adds to the effectiveness of any shield you may have but also your damage against shielded enemies. Strong alternatives include Siphoning Spells and Juggernaut.
Gifted Rider, Steed's Blessing, Rationer and Liquid Efficiency are the usual Craft slottables many use but War Mount can be a good one should you be more wary of people trying to dismount you.
Introduction (Update 30, Blackwood)
The Detonation build is your typical Magicka Nightblade PvP bomber that lurks around large groups of players in Cyrodiil or Imperial City, most likely in Champion Points enabled campaigns. While certainly you can bomb in no-CP environments including Battlegrounds, it's not really recommended.
And yes while you can also bomb a player and not a group, that is not also recommended unless you really need to get them off your back or maybe they teabagged you earlier.
Beyond the build, it takes considerable patience, getting used to the skills' timings and adaptability to circumstances that may be plagued with lag or high ping spikes during prime time PvP to pull off bombing. To see gameplay or demonstrations of how to bomb, refer to the above video.
Recommended races for this build are Khajiit, Breton, Altmer and Dunmer.
Recommended Mundus Stone is The Shadow.
Recommended potions for this build will either be a Spell Power Potion (AP bought or Crafted) or an Essence of Immovability.
Recommended food for this build will be Ghastly Eye Bowl or Witchmother's Potent Brew.
Vampire can certainly be an option depending on how you want to go about things.