All There in the Manual: The side story known as "History of Niefelheim" explores many events that happened before the start of Heart Core, sutch as the death of Lilium, the birth of Ame and the war between demons and beastmen. Some of the events in the story has been stated to appear in the comic.
Anime Hair: Considering that the webcomic has an artstyle based on anime, this shouldn't be a suprise to anyone.
Badass Beard: Warlord Karlsen, a Beastman who looks like Augus if he dressed up as a paladin. Sure, he probably died just as fast as he appeared after Volaster blew the Beastman capital into ashes (Making his total comic time 4 pages), but he is so far the only mortal character who has been seen causing significant damage against a demon. Not just any demon, but an Overfiend!
"...a demon never truly dies...but they never truly reproduce either. It's rare, but when a demon decides to renounce their life, an egg with their reincarnation appears...This egg will also have attributes from another demon they've been in contact with. Within two years' time, the new demon will require the originator's Heartcore, effectively relieving them of duty."
Black Eyes of Crazy: The whites of characters' eyes turn black when fighting. Particularly powerful demons like Lilium and Volaster have permanently black eyes.
Blood Magic: Played with. Salamander demons such as Carval are said to have fire flowing through their veins, and...well, Word Of God says you shouldn't make them bleed.
Bond Villain Stupidity: Royce is guilty of this. Considering his power, he could have easily killed Ame at the very begining of the first chapters, but he thought sending his weaker servants against her in order to make them compete for a position of power would be more entertaining.
Curb-Stomp Battle: The only two Overfiends other than Royce that actually managed to put up a fight against Ame during her escape from Asgard were Teodora and Slade. The rest of them stood little to no chanse.
wile on the topic, the only thing that saved Ame and Suko during their fight with Royce, was the fact that Lilium possesed Ame, giving her the power to make it an even fight. Even then, it served mostly as a distraction just to give them the opening to flee once and for all.
And then there was Volaster vs the entire Beastman capital. Guess who won.
Dark and Troubled Past: Having his mother die of illness, growing up poor and getting his ass kicked for stealing wile only being a mortal child was enought to cause Volaster to blow up his home wich in turn caused him to become a demon and getting found by Royce. Not that Ame cares, of course.
Darker and Edgier: Would you believe that the characters trying to brutally murder each other used to be part of a comedic, gag-a-day webcomic? Or that even before THAT, Ame used to be a Sonic the Hedgehog fan-character before being remodeled into a succubus?
Dark Is Not Evil: The heroes of Heartcore, although you may say otherwise at some points.
Deader Than Dead: The only way to kill a demon is to either destroy its head or rip out their heartcore, Word Of God says the latter is easier.
Demonic Possession: This happens a lot, for example Lilium possessing Ame and Volaster possessing Carval
Royce, as well. His most well-known form is just him possessing the dead body of a beastman.
Fanservice: Considering that one of the main characters is a succubus, this trope is bound to happen a couple of times through the series.
Fat Girl: While not necessarily fat, Emerald is fairly chubby. This is mainly the result of influence from the character's original creator Cory Holmes, for whom large-bodied girls is something of a kink.
Fetish Fuel Station Attendant: We could be here all night going through the list, but suffice to say that none of the female characters' designs were an accident.
Fish out of Water: Ame has some trouble adjusting to her exile in human society. It's also been said that demons are easily overwhelmed by human sights and sounds, which is why Beastmen are practically required to assist them as tour guides and bodyguards.
Good Wings, Evil Wings: Demons have wings of various shapes and sizes, from Ame's itty-bitty little red wings, to trueform!Royce's badass prismatic wings.
Gory Discretion Shot: Sukina's face gets seriously messed up by Ame while she is under Royce's spell. We never see the injury, though, as the afflicted side of Sukina's face is cleverly hidden behind Ame's head after she picks her up.
It Is Pronounced Tro Pay: Amethyst's shorthand, Ame, was originally derived from the Japanese word for rain. Word Of God says Ame's nickname is not pronouned "Ah-May", but instead is pronounced like "aim".
Master of Illusion: Ame has shades of this. Exhibit A: assuming the form of a helpless human about to be harassed by a couple of thugs, then opening a can of whoopass on them.
The Merch: In-universe, Syranon Glaed is famous enough in New Ayers to have merchandise based on him due to his popularity.
Mook Horror Show: The first chapters can be summed up like this for Teodora, Emerald, Carval and Goristro. Sure they might be powerfull demons, but let's see YOU try to fight the daughter of the demon who rules Asgard without crapping your pants!
Morphic Resonance: Even though Ame has the power of shapeshifting into other women and can shape her clothes any way she pleases, her hair must have the same shade of pink or green that she normally has. Also, her clothes must be either the same shade of black, red or white that her normal outfit has. Thankfully for her, she can mix these colors in any design she pleases.
My Name Is Not Durwood: Lutz appears to have this problem, with people merely referring to him by his title rather than by his given name.
Name's the Same: Slade has no relation to Deathstroke. This is lampshaded in the chapter one parody. Word of God says Slade was actually named after the Rat Thief character "Slade" in Shining Force II for the Sega Genesis.
Pyro Maniac: Nearly all of Carval's powers include fire in one way or another. To say that he really enjoys using them, even in public areas of the human world where everyone can see it, would be an understatement.
Red Oni, Blue Oni: Amethyst and Royce are the Red to Teodora and Asmodai's Blue, both in attitude and color.
Reincarnation: According to Volaster, demons do not die, but come back as a different form. When demons decide to give up living, an egg carrying their new form appears bearing attributes of the original demon and another demon the progenitor had contact with. Wait two years, introduce the original demon's Heartcore to the egg and voila! The new incarnation lives on, carrying the consciousness of its original form.
In short, Carval is the reincarnation of Volaster, and to an extent Ame is the reincarnation of Lilium.
Many characters' names harken to classic Sega. The family name Lashiec, for example, comes from the actual translation of the name "Lassic", the villain of the first Phantasy Star. Likewise, Lutz is named after the esper in the same game. Then there is also the afformentioned Slade...
Villainous Breakdown: Volaster didn't take it well when Ame decided to press all of his Berserk Buttons (interrupting his childhood story and claim that Royce doesn't care about him). In fact, he took it so badly that he tired to destroy the dome that protects New Ayers, the last human settlement.
and in "History Of Niefelheim" Royce had a big one when his arm was cut off, butt also had an even bigger one when he found out that Lilium died and gave birth to Ame, making him unable to eat her heartcore and thus putting a stop to his plans for greater power.