This doesn't exist in D&D 5e (yet, at least).
As of June 2018, a search of D&D Beyond's magic items section shows no brilliant energy weapon in the D&D 5e rules. There's also none in the homebrew rules, except for one particular very powerful artifact weapon that isn't really equivalent.
It's not necessarily going to be made (officially)
As GreySage wisely pointed out in a comment, recalculating armor class on the fly is awkward. At least, you have to open up the book to find out what bonus certain armor grants, and at worst you've got a monster with a non-standard armor.
This wouldn't have been unusual in D&D 3e, where creatures regularly had multiple armor classes depending on which bonuses the player's attack can ignore (touch AC, flat-footed AC, flat-footed touch AC...), but in 5e this has been simplified away. For these reasons, I wouldn't hold my breath for an official 5e conversion of brilliant energy weapons.
The first thing is that D&D 5e now sorts items into rarity, but does not price them. This makes it considerably easier to add a new magic item. It would convert something like this:
Brilliant energy weapon
Weapon (any), rare (requires attunement)
This magic weapon has its blade or striking surface transformed into light. It glows as a torch, and passes through non-living matter. Opponents who wear armor or use a shield do not gain the benefit of those items against attacks by this weapon, even magical armor. It deals no damage to objects or non-living creatures, such as constructs and undead.
However, player-crafted magic items aren't a standard thing any more in D&D 5th edition. It's purely at the DM's discretion, and there's no such thing as a Craft Magic Arms and Armor feat any more. Thus even if you had the stats for a brilliant energy weapon, there's no guaranteed way for the player character to make it, unless you ask your DM nicely.
Another way might be to create a spell which turns a weapon into brilliant energy, and has the same effect. Thematically, a creating an "ethereal arrow" might fit the warlock well, though in terms of a 3e sorcerer's spell selection, you're closer going with the sorcerer or Eldritch Knight (5e's fighter build which can cast some arcane spells).
It might be as simple as a powered-up higher-level variant of true strike which lasts for several arrow shots, but doesn't affect undead or constructs. Again, homebrew spells are at the DM's discretion.