I believe that is a spell, not a class ability.
In 4e, powers are class specific. They're class features.
But why does that even matter?
@ thread:
Now, I do think the warlock is close enough that you can use it to fairly easily build an actual Hexblade class out of it, with variants of all the patrons but with more weapon-combat oriented abilities, less advanced Spellcasting in exchange for extra attack and martial weapons, higher HD, built in option of defense mechanics (ie either armor proficiencies or level 1 armor of shadows), and then tweak until you’ve got a proper Hexblade that still has choices within that concept. Thematically similar to the 4e HExblade, but designed fully to be a good 5e class.
Likewise the Artificer, Paladin, and Ranger can be used as inspiration to build an arcane half-caster on a more traditional chassis.
The Bladesinger can be beefed up and rebuilt as if the Bladesinger stuff was the base class, and probably be pretty cool.
Or we could take all the kinda-gish options in the game, dissect them, look at older editions and D&D clones and 3pp/homebrew at places like DMsGuild and the homebrew subreddits, and build a fully fleshed out swordmage (or other name so folks don’t insist on trying to recreate the 4e class directly rather than just making a good 5e class) with a secondary option like warlocks and UA clerics have that determines whether your Aegis is personal, protective, or something else.
So, what I'd probably do if I had a player that really wanted to play a swordmage:
Basics: d10 HD, martial weapons, light armor
Spellcasting at level 1, either like artificer or a half-caster pact magic variant. Same spell slots (half the slots just wouldn't be functional), but half caster spell level progression. 1 Spell slot and 2 cantrips is doable at level 1. You have a spellbook like a wizard. It's in the name.
Rather than better armor proficiency and shields, you get something like the bladesong, with a bonded weapon that you can summon as a bonus action, and returns to you if you throw it as an attack. This is your big tier 1 class feature.
Secondary choice (like pact boons) at level 1, which are each an Aegis but not all make you a "defender", and this is tied in with how your primary feature functions. The Aegis has active and limited abilities that probably come online at level 2, and possibly upgrade in some way or scale in number of uses as you level in the class. ie, your punishment is probably PB/LR or 1/SR, rather than at-will, but your mark ability is at-will.
Drop spellcasting back to level 2 if necessary, or maybe have cantrips and rituals but no spell slot at level 1, to keep in balance. The aegis and swordbond kinda need to be level 1, in some form.
Maybe the choice at level 1 is between different marks, actually, and your defensive magic is just called your aegis. Mark of Retribution lets you attack enemies who attack your ally, and later you gain an aura that automatically deals damage to enemies who attack allies within the aura. Mark of Death is like Hexblade's Curse, increasing your crit chance and maybe giving an extra way to get advantage. The idea is you get some swordmage juice at level 1, and your divine smite type power boost comes in later but ties into the level 1 choice of mark or aegis or whatever.
Level 2 something like Vestiges or some such, basically overlapping with Invocations and Infusions. Some key off your aegis, so you can hard-focus on your mark-punishment tankiness, or the aegis choice that makes you more mage-like, or the one that makes you harder to pin down and play almost like a 4e Avenger. others are the sorts of things warlock invocations do or are special items or weapon enchantments that make your bonded weapon gain extra magic weapon traits, etc.
level 3 is your actual subclass, based on esoteric orders. This is separate from the aegis and mark dynamic, and gives bonus spells, and features, that promote differnt playstyles and thematic identities but all tie into the idea of mysterious orders of esoteric knowledge blending magic and martial skill. Swordmages are hermetic occult nerds, basically.
You get extra attack at level 5, and an aura that is flavored as extending your aegis to protect allies or harm enemies or whatever at level 6 or 7
Because you are getting new spell levels at half the rate of a warlock, there is more room to play in the non-spell space of the class, but spells shoulnd't be neglected. New spells that support the idea of a Swordmage (and stealth upgrade other gish options with access to arcane spells in the process) are key.
Spell ideas:
Throw sword, it's flies in an arc applying your melee weapon attack to 3 enemies, plus 1 for every spell level above 2nd.
Throw sword, it explodes dealing magical slashing damage and potentially blinding enemies, then reforms in your hand.
Sword turns into a streak of lightning, dealing okay damage in a line to a target, you teleport to the target, and your next attack against them deals extra lightning damage.
Your sword's shadow animates and flies around stabbing people (duration 1 minute with concentration)
Your sword duplicates over and over again creating a circle of whirling blades around you
Your weapon strikes the ground, sinks in and disapears, and then the ground around you grows spikes that cut and pierce for minor damage, and slow enemies movements, but ignore allies. (higher level spell here)
Aegis/mark ideas:
Your marked enemy grants advantage to your allies' attacks
You can move more or teleport or something that makes it impossible for a marked enemy to escape you
You carry a spellbook in your off hand, and gain extra spells, or similar, but either way you can use the book as an arcane focus and don't need a free hand to cast somatic spells while weilding it.
Your mark harms the enemy whenever they move more than 10 feet in a direction that takes them away from you, or hit a creature other than you.
You can teleport to your marked target if they are more than 5 feet away from you at the start of your turn, with distance scaling by level.
I'm tired, but I had ideas earlier for tying the choice feature into your not-bladesong.
Very much a hybrid warlock/monk/4e avenger/4e swordmage class.
Maybe 3-5 level 1 choices, basically promoting the Defender, skirmisher, lurker, swordMage, and the duelist.
Orders are things like Order of The White Well - fey themed spell-blade duelist meets knight errant on a quest from a magical Lady met in the wilderness.
Order of The Shadowed Dawn - Creepy shadow cultist swordmaster with stuff like animating your shadow to fight beside you.
