Gravelgill Duo 

Creature — Merfolk Rogue Warrior
$0.09 #29696 0.38 18.3 years
Whenever you cast a blue spell, this creature gets +1/+1 until end of turn.
Whenever you cast a black spell, this creature gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)
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God-Pharaoh's Faithful79%
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"Whenever you cast a blue, black, or red spell, you gain 1 life."
Hydro-Man, Fluid Felon78%
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"Whenever you cast a blue spell, if this card is a creature, he gets +1/+1 until end of turn."
Gearsmith Guardian78%
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"This creature gets +2/+0 as long as you control a blue creature."
Briarberry Cohort78%
$0.17↑#260340.00↓18.3 years
"This creature gets +1/+1 as long as you control another blue creature."
Wingrattle Scarecrow78%
$0.55↑#11261↑0.50↑18.3 years
"This creature has persist as long as you control a black creature."
+1 more matching line
Emberstrike Duo75%
$0.10↑#299950.27↓18.3 years
"Whenever you cast a black spell, this creature gets +1/+1 until end of turn."
Quirion Dryad75%
$0.10↑#250140.3525.6 years↑
"Whenever you cast a spell that's white, blue, black, or red, put a +1/+1 counter on this creature."
Thistledown Duo74%
$0.08↓#304570.07↓18.3 years
"Whenever you cast a blue spell, this creature gains flying until end of turn."
Minotaur Tactician73%
$0.06↓#318710.00↓25.3 years↑
"This creature gets +1/+1 as long as you control a blue creature."
Merrow Bonegnawer73%
$0.20↑#16837↑0.86↑18.1 years
"Whenever you cast a black spell, you may untap this creature."
Merrow Levitator73%
$0.20↑#254100.06↓18.1 years
"Whenever you cast a blue spell, you may untap this creature."
Glade Gnarr72%
$0.05↓#315110.53↑25.3 years↑
"Whenever a player casts a blue spell, this creature gets +2/+2 until end of turn."
Ashenmoor Cohort72%
$0.14↑#313120.27↓18.3 years
"This creature gets +1/+1 as long as you control another black creature."
Scornful Aether-Lich72%
$0.17↑#313550.00↓17.6 years
"
: This creature gains fear and vigilance until end of turn."
Questing Druid // Seek the Beast70%
$0.36↑#8502↑0.333.0 years↓
"Whenever you cast a spell that's white, blue, black, or red, put a +1/+1 counter on this creature."
Ana Sanctuary70%
$0.18↑#18382↑0.00↓25.3 years↑
"At the beginning of your upkeep, if you control a blue or black permanent, target creature gets +1/+1 until end of turn. If you control a blue permanent and a black permanent, that creature gets +5/+5 until end of turn instead."