Ttharg Krixkrux

Ttharg continued to move towards the centre of the room. Sparks and lightning flashed from the mysterious arcane equipment; lightning Ttharg could use.

Using his free hand, he grabbed hold of his Amulet of the Devout and made a prayer to Bahamut for control over the arcane storm raging within this platform.

He then pointed at the Mage and directed a bolt of lightning to strike at her. As the bolt struck, Ttharg directed the wrath of Bahamut into the strike, hoping to deal a significant blow.

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Move Action to move 30ft towards the centre.
Main Action to cast Call Lightning at 3rd Level, centred in the middle of the room. As part of the spell, I'm targeting the Mage with a bolt from the storm.
Amulet of the Devout gives me a +1 DC to spell DC, but Half-Cover gives the Mage +2 to Dex saving throws (I'll factor that into the roll). Normal spell DC is 14, so DC is 15. Saves for Half.
Mage Dex Save vs 15: (1d20+2)[16]+dex mod.
Regardless of whether she passes her dex check, Ttharg will use a charge of his Channel Divinity to deal maximum damage, so 40 Lightning Damage or 20 if saved.
Finally, the Mage is pushed 10ft away from me, hitting the deck.