When you reach third level, you gain limited spellcasting. See the Eldritch Knight or Arcane Trickster tables in the Player's Handbook for spells per day and spells known.
You learn three cantrips from the Smiling Trickster spell list. You learn one more at tenth level.
Charisma is your spellcasting ability.
Also at third level, you're able to set little tricksy traps with your spells. When you cast a spell, you may choose to have the effects delayed, for a number of rounds no greater than your proficiency bonus. For instance-if you cast Guidance at third level, you could target an ally, wish them well with your spell, and have the effects only begin two rounds later. Or, you could cast Greater Invisibility at twentieth level, choosing to delay it for six rounds, go on stage, give a good speech, and then turn invisible without appearing to have cast a spell.
If the spell requires Concentration, you must Concentrate on the spell immediately when it is cast, and maintain Concentration as normal or lose the spell. Note that the duration does not start until the spell has actually been cast.
At ninth level, whenever you cast a spell that targets one or more allies, you may choose to grant them one of two bonuses. YYou may either give them THP equal to the spell's level plus your proficiency bonus, or a bonus on their next d20 roll equal to 1d4 plus the spell's level. Whichever bonus is granted, it fades after one minute. Cantrips, for this ability, count as level zero.
Eyes On Me!
At thirteenth level, you may put on a display that draws eyes towards you. As a bonus action, you may make a dramatic flourish that catches the attention of one or more creatures you choose within 60'. If they fail a Wisdom Save against your spell DC, they are compelled to focus on you until the start of your next turn, as their eyes are inexorably drawn towards you. They suffer disadvantage on any attack rolls that target someone other than you, and any saves they force against anyone but you are made with advantage, unless the same ability that forces the save also includes you. (For example: a Fireball that includes you in the AoE would have saves made normally, but one that does not include you would have all saves made at advantage.)
This ability lasts until the start of your next turn, or until you break line of sight to the creature(s) affected. Note that the effected creature(s) merely closing their eyes is not sufficient to break this ability-there must be an actual obstruction between you and them.
At seventeenth level, you are able to act as a medium for casting abilities. This has two abilities.
Firstly, you are able to gain spells from others. You can gain a number of spell levels equal to twice your proficiency bonus total-any attempts to gain spells past this limit fail. Over the course of a long rest, another friendly caster may bequeath to you spell slots and spells known, up to your limit. For example-a friendly Wizard could give you Meteor Swarm along with a 9th level slot, and Fireball along with a 3rd level slot. These are added to your spells known and slots available until you finish your next long rest. The Wizard would not lose the spells known, but would lose the spell slots given to you.
Secondly, you are able to give spells to others. The recipient can gain a number of spell levels equal to their proficiency bonus-any attempts to give spells past this limit fail. Over the course of a long rest, you may give the spell slots and spells known, up to their limit. For example-you could give Bless and a 1st level slot, as well as Expeditious Retreat and another 1st level slot to a friendly 3rd level Fighter. These are added to their spells known and slots available until they finish their next long rest. You lost the slots given, but not the spells known.
Both abilities may be performed over the same long rest. Cantrips may not be given or received. And finally, the spellcasting stat used is that of the original caster-in the first example, the Smiling Trickster would use the Wizard's spellcasting stat (Intelligence) instead of Charisma, for the given spells.
Smiling Trickster Spell List
Spoiler: 1st Level
Speak With Animals
Spoiler: 2nd Level
Gust Of Wind
Pass Without Trace
Zone Of Truth
Spoiler: 3rd Level
Spoiler: 4th Level
Freedom Of Movement
So what I'm least sure is the spell list. It's VERY utility and support heavy, with minimal direct damage options. Still-feels a little cluttered.
Any advice or critiques, gladly appreciated!
Talking to one of my friends, they're concerned about the level nine ability. They suggested a replacement, which I'll write below:
At ninth level, when you cast a spell, you may attempt to hide that. If the spell uses material and/or somatic components, use a Sleight of Hand check. If the spell uses verbal components, use Deception. If it includes both, use both checks. When you cast a spell, you may roll the appropriate check(s) against another creature's passive Insight. If you beat their passive Insight, they do not realize that you just cast a spell, unless they have other means of perceiving it.