Dracula's Body Parts
Although fans (and critics, and casual players) are divided on the legacy of Castlevania II: Simon's Quest within the series, there can be no doubt that at least one motif from that NES sequel has stuck around throughout the series. That would be the finding, and collecting, of Dracula's Body Parts (also known as Dracula's Relics). Various games, from Castlevania II and beyond, have tasked the hero with gathering these parts to clear a path to the end of the adventure.
Why, though, are these parts so important? Frankly, we can point to two major factors that have kept these items coming back, time and again. The first is their direct connection to Dracula himself. While other relics that have appeared have often duplicated powers from other games (think the Leap Stone and the Gryphon Wing, among many other that provide a powerful high jump), these were never associated with the main antagonist of the series. But collecting the actual parts of Dracula lends a visceral quality. You know these relics have power to them without even needing to be told. They're of Dracula!
From a pure game play mechanic, too, it makes sense to keep them around. Going out into an adventure and collecting items, whatever they may be, has been a part of video gaming basically for as long as games have existed. With Dracula's Body Parts the series had a ready-made set of items that naturally gated and enhanced the progression. Five items (or sometimes six) that could be reliably used to set goal posts in the game, again without any needed explanation. Once you saw a part of Dracula floating there, ready to be collected, the gamer already knew what to do (and what it implied).
Thus, with their inclusion in one game (and the second game overall of the series), Dracula's Body Parts cemented their own legacy within the series. They became an integral part of the fabric of Castlevania, an offal (but not awful) part in that tapestry.
About Crosses:
The Powers of the Body Parts
First appearing in Castlevania II: Simon's Quest, Dracula's Relics were a required collectible within the game, tied directly to the story itself. The hero, Simon Belmont, had been cursed by Dracula and was forced to find, and collect, the body parts of Dracula. Only by combining those five parts -- Eyeball, Nail, Rib, Ring, and Heart -- at the ruins of Dracula's Castle could Simon break the curse. Of course, by assembling the parts Simon inadvertently resurrected Dracula and the hero had to battle the vampire once again. If he was fast enough in his quest, Simon would survive the curse and live, continuing the Belmont legacy for centuries to come.
While the body parts were required for the end game, they also provided necessary abilities for exploring Romania, too. The eyeball could reveal secrets, such as a floating platform in the first mansion Simon came across, allowing access into that mansion. The rib acted like a shield, blocking projectiles. The nail enhanced Simon's whip, allowing the hero to break certain bricks. The heart could be shown to the Ferryman, who would then take you to a secret place in Romania that contained further progression forward. Finally, the ring would open the way into the castle ruins.
It would be some time until the relics appeared again, but they did (naturally enough) in the next non-linear adventure game in the series: Castlevania: Symphony of the Night. Produced by Koji "IGA" Igarashi, this game was a loving ode to classic games while adding plenty of new mechanics into the series. One of those lovely homages was the return of Dracula's Relics. Collecting these five -- nail, ring, heart, rib, and eye -- from bosses in the Inverted Castle portion of the game (each guarded by a re-imagined classic boss from the very first Castlevania) would then grant hero Alucard access to the chamber at the very center of the Inverted Castle where he could battle his father for the fate of the world.
While the relics served a similar purpose for exploration as they did in Castlevania II -- they were required to beat the game -- they also bestowed power onto Alucard as well. The ring increased his Intelligence (INT) by five, the heart nullified curses, the rib enhanced his Constitution (CON) by ten, the tooth increased his Strength (STR) by ten, and the eye increased his Luck (LCK) bu ten. These helpful increases made exploration of the castle that much easier.
For IGA's next foray into Metroidvania exploration, Castlevania: Harmony of Dissonance, the body parts made their return. As before, collecting the relics was required to progress into the castle center and fight the final bosses of the game. This time around, however, there were six items to collect, with the fang joining the nail, eye, ring, heart. and rib. And, of note, the final form of Dracula in this game was a weird amalgamation of enlarged versions of all six body parts. It's really weird looking.
As in Symphony of the Night, the relics in Harmony of Dissonance also conveyed some small powers onto the hero. The eye negated curses, the heart nullified petrification, the rub took care of poison, the nail increased hero Juste Belmont's STR, the fang increased his Defense (DEF), and he ring increased his LCK.
And then, with Castlevania: Symphony of the Night the series introduced the Grand Cross. Prior to this game, hero Richter Belmont could perform an item cross, sending cross spiraling out from him in a rotating pattern. Instead of that kind of "crash" though, or even a normal cross usage, Symphony gave Alucard a Grand Cross (although still referred to as just a "cross" in this game). Standing still, Alucard would become engulfed in a beam of light, giant crosses spiraling upwards around him, damaging all enemies on screen. It was a powerful attack, although it did use up a fair number of hearts.
Finally, the relics are once more mentioned in Castlevania: Order of Ecclesia. The powerful bits of Dracula were given to the Order for safe keeping, with the thought the group would prevent the resurrection of Dracula. However, the leader of the order, Barlowe, became corrupted by their evil influence and, in the end, actively worked to resurrect the Dark Lord. It was up to heroine Shanoa to fight her way to the resurrected Castle Dracula and fight the demon once more.
Miscellaneous Trivia:
It is worth noting that while the fang (which was eventually included in Harmony of Dissonance) does not officially appear in Castlevania II, a graphic for the relic can be found in the code of the game. That implies it was originally intended as one of Dracula's parts and was just made official in the later entry in the series.
In Other Media:
In the Elder Scrolls V: Skyrim DLC, Dawnguard (which had a lot to do with vampires), a room in Castle Volkihar can be seen to contain the relics of Dracula. This includes a display case for a ring, a nail, a heart, an eye, and a rib cage. A cute little reference showing the legacy of Castlevania II goes beyond just the main Konami series.