The idea is first turn cast darkness using sorcery points so you can see through it (shadow magic) then action surge to eldritch blast. Then you use the rogue cunning action to hide. In following turns use action to blast and bonus action to hide. Hiding in darkness means you cannot be targeted by spells or attacks except by enemies that can see through darkness or are immune to blind – of which there are very few. With Armor of Shadows and 16 Dex you can get 16 AC, which is OK and you will have Shield spell in the exceptionally rare times you need it. Your main weakness are AoE from enemies but most of these are Dex saves and starting as Rogue gives you Dex Save Proficiency.
Attacks will be made with advantage (due to darkness) and thanks you champion, cloak, knife, bow and spell sniper you score a critical hit if you roll a 15 or higher on the d20, so about 1/4 of your eldritch blast beams will be crits and you shoot 3 beams a blast. Whenever you do crit your beam deals an additional 1d10+2d6 on top of the 1d10+12 it already does thanks to potent robe and agonising blast. It also has a chance to apply Frightened to the target and enemies near them thanks to Great Old One.
Half-Illthid can net you Freecast, expertise in Persuasion, Intimidation and Deception, Flying, and other good things.
I chose Duergar so it is easier to start fights in advantageous positions or to escape fights. The invisibility can also help you start fights with Surprise which can keep you safe in the first round until your turn.
Spell choice isn’t a big deal, choose whichever you want beyond Eldritch Blast.
