"Umbral Horror", known as "Umbral" (アンブラル Anburaru) in the OCG, is an archetype used by Vector in the Yu-Gi-Oh! ZEXAL anime. It is composed mostly of weak DARK Fiend monsters who revolve around Special Summoning other "Umbral Horror" monsters for quick Xyz Summons.
"Umbral Horrors" have unique abilities to quickly swarm the field to allow for quick Xyz Summons. Their DEF at 0 also make them an ideal target for cards such as "Masked Chameleon", "Secret Sect Druid Dru", that can revive most of them from the Graveyard at the expense of having their effects negated. However this can also allow for successive and constant Summons of Rank 4 Xyz monsters and Level 5 or Level 8 Synchros, thus being capable of Summoning powerful monsters with ease. "Recurring Nightmare" can be used with the "Umbral Horror" monsters, principally "Umbral Horror Ghoul" or "Druid Dru" and another "Umbral Horror" for an instant Rank 4 monster. Cards such as "Allure of Darkness" can be used easily for draw power. Furthermore considering the 0 ATK nature of "Umbral Horrors", including "Ghoul" through its own effect, "Reptilianne Vaskii" can be Summoned with ease, allowing potent field control through its effect.
Because of their effective ways to Summon Rank 4 monsters, one can afford Rank-Up plays with the Deck. Also if one uses the effect of "Umbral Horror Unform", it is entirely possible to Xyz Summon a Rank 4 that requires 3 materials with relative ease, and through Rank-Up one can easily Summon powerhouses such as "Number C39: Utopia Ray V", "Number 104: Masquerade" and its evolved form "Number C104: Umbral Horror Masquerade" and also "Number 105: Battlin' Boxer Star Cestus" and "Number C105: Battlin' Boxer Comet Cestus" for massive field control and brutal damage potential.
Due to the lack of internal support and their own boss (bar perhaps "Number 104"), Umbral Horror Decks tend to have small restrictions on their deck building. Therefore one can place pretty much as many techs of their preference as they want, without hindering them too much. The same is true for their Extra Deck, being capable of running almost any and all generic Rank 4 Xyz Monsters and Level 8 Synchro Monsters, thus the player can have as many options as they want to incorporate for any situation, making the deck incredibly versatile on its plays and responses.
As explained above, "Umbral Horrors" have low ATK and DEF, rendering them quite weak by themselves and thus likely to be destroyed. Furthermore effects such as that of "Umbral Horror Unform" needs it to ram itself into an opponent's monster, thus likely causing a substantial damage to one's Life Points. Most of the external support such as "Masked Chameleon", "Druid Dru", "Debris Dragon", and "Recurring Nightmare" focus exclusively on the Graveyard; therefore, cards that block the Graveyard such as "Necrovalley", "Macro Cosmos", or "Dimensional Fissure" can be downright fatal to the Deck. All "Umbral Horror" monsters and "Druid Dru" are DARK, so cards such as "Shadow-Imprisoning Mirror" can block all of those monsters, leaving few options available. Cards that prevent effects such as "Skill Drain" can easily block "Masked Chameleon", "Druid Dru", and "Debris", thus preventing any revival plays from those monsters. "Chain Disappearance" can annihilate most "Umbral Horrors" and render them useless through banishing. "Doom Donuts" can also be used against "Umbral Horrors", although it is more of a short-term annoyance rather than a definitive threat.