Partly updated for Enshrouded 1.0— 1.0: the unlock cost; the rest is pre-1.0 wiki data

Thunder
All shock damage is increased by an additional 10%.
Where Thunder sits on the 1.0 tree
In the Mage area. The cheapest way in from a starting point buys 8 nodes for 11 points, this one included (one rank each).
Builds that use Thunder
3 preset builds tagged THUNDER — defining picks first, then ranked by tier (S → A → B).
- Battlemage Hybrid⭐ DefiningHybrid
Half Warrior, half Mage: Warrior's Path, Slasher and Heavy Plates for the one-handed sword, Thunder at full rank with Mage Apprentice and Wizard for the shock wand, Battle Heal to sustain.
- Wand ArchmageMage
A Mage built on the wand: Wand Specialization, plus Aerial Wand Attack and Wand Evasion Attack, both new in 1.0. Wand Mastery, Sting and Arcane Proliferation add wand damage, Wizard adds magic crits for Chain Hit, Mass Destruction and Blast and Cast, and Thunder covers the shock wand.
- Endgame MetaHybrid
Every one of the 212 points: two-handed Warrior (Barbarian's Path for the axe, Titan Edge for the greatsword, Slasher, Veteran, Relentless, Whirlwind Crescendo) in Heavy and Shiny Plates, Thunder and Wand Mastery for a shock wand, Blink, and the full Healer line. Pyromaniac is not on the 1.0 tree.
Related Mage skills
Same archetype — likely candidates to slot next to Thunder in a build.
Arcane Deflection1pOn a successful parry, gain 20 mana.
Counterstrike1pThis magical attack can trigger other skills.
Eternal Spark1pValues in the 1.0 files: 15% per rank, 3 ranks.
Fire Resistance1pIncreases your Fire Resistance by 5% which reduces the amount of Fire damage received.
Healer1pValues in the 1.0 files: 15% per rank, 3 ranks.
Healing Revive1pValues in the 1.0 files: 8% per rank, 3 ranks.
Ice Resistance1pIncreases your Ice Resistance by 5% which reduces the amount of Ice damage received.
Intelligence1pIncreases your Intelligence attribute by 1 Increases Magic damage by 5% per Attribute Point
Sources: game files 1.0 + Keen Games 1.0 patch notes (changelog, combat changes) for the unlock cost; enshrouded.wiki.gg (pre-1.0) for the description and effects.