Abyssal Nocturnus 
Creature — Horror
$0.68 #20128 0.27 20.6 years
Whenever an opponent discards a card, this creature gets +2/+2 and gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)
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Cards similar to Abyssal Nocturnus expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Face of Fear79%
$0.15↓#31485↓0.00↓24.9 years↑
"
, Discard a card: This creature gains fear until end of turn."
, Discard a card: This creature gains fear until end of turn."
Sangromancer75%
$0.69↑#2002↑0.3515.6 years↓
"Whenever an opponent discards a card, you may gain 3 life."
Liliana's Caress75%
$5.31↑#2412↑0.51↑16.1 years↓
"Whenever an opponent discards a card, that player loses 2 life."
Hooded Kavu74%
$0.21↓#30724↓0.2525.9 years↑
": This creature gains fear until end of turn."
: This creature gains fear until end of turn."
Tourach, Dread Cantor73%
$0.47↓#6178↑0.56↑5.3 years↓
"Whenever an opponent discards a card, put a +1/+1 counter on this card."
Confessor73%
$0.20↓#191690.43↑24.9 years↑
"Whenever a player discards a card, you may gain 1 life."
Fell Specter72%
$0.34↓#4878↑0.41↑8.1 years↓
"Whenever an opponent discards a card, that player loses 2 life."
Geth's Grimoire72%
$8.54↑#2318↑0.54↑22.6 years↑
"Whenever an opponent discards a card, you may draw a card."
Nath of the Gilt-Leaf70%
$3.01↑#12831↑0.40↑18.9 years↓
"Whenever an opponent discards a card, you may create a 1/1 green Elf Warrior creature token."
Spirit Cairn70%
$0.12↓#209810.00↓24.3 years↑
"Whenever a player discards a card, you may pay 
. If you do, create a 1/1 white Spirit creature token with flying."
. If you do, create a 1/1 white Spirit creature token with flying."
Vulturous Zombie70%
$0.32↓#163480.2520.9 years
"Whenever a card is put into an opponent's graveyard from anywhere, put a +1/+1 counter on this creature."