SPOILERS ARE APLENTY

We shall be looking into perhaps one of the most enigmatic characters of Elden Ring.

Before proceeding know that this article delves into possibly one of the most impressive secrets of the game.

Popular Youtuber and expert on Elden Ring information as well as data, Zullie the Witch, posted a video recently covering an important discovery in regards to Placidusax, one of the game's optional if last bosses.

She remarked that his title and weapon, a tail-formed sword that the player gets from retrieving the "memory", i.e. reward, from said dragon and exchanging it, could refer to the old tradition in From's games of having the player chop off the tail of a dragon and receive a sword or weapon from them.

Furthermore, the name of both Placidusax and that of a weapon of a Dark Souls 1 boss, the Gaping Dragon's Dragon King Greataxe would hint at an homage linking these two and the tradition.

We will be looking at the flavour text from three items appertaining to Placidusax in the game, the "Dragon King Cragblade", "Placidusax's (sic) Ruin" and "Remembrance of the Dragon King."

Prior to beginning a small note on the name of the dragon himself, in Japanese script it is written プラキドサクス, purakidosakusu, i.e. implying that the name is pronounced as Classical Latin, rather than having the "c" as an /s/, but as a /k/, therefore it's "pla-kee-do-sax" and not "pla-see-do-sax." His name consists of two parts: Latin placidus meaning "calm, still, peaceful, gentle, placid", especially the senses of "still", "calm" and "placid" are apt due to his dormant nature most of the game, only slowly waking up to fight the player once they enter his mausolaeum; saxum, the second part, literally means "rock, stone" referring to his body's literal petrified state. Hence, this suffix "-sax" repeats itself in the names of other boss dragons - hearkening back to that age. Minerals, rocks, crystals, alchemy and so on figure strongly and symbolically in Elden Ring.

Dragon King's Cragblade (竜王の岩剣, Ryuu'ou no ganken, Dragon King's Boulder/Rock Sword):

The description is as follows in the official English version:

Piercing Gravel Stone sword containing primeval lightning. A portion of the Dragonlord's power, gained from a remembrance.

This weapon commands great power over the paltry, mortal dragons of today.

And the original Japanese version:

原初の雷を宿したさざれ石の刺剣
追憶から得られた、竜王の力の片鱗
永遠なき、卑小な竜の末裔たちに高い威力を発揮する

Gensho no kaminari o yadoshita sazareishi no shiken. Tsuioku kara erareta, ryuu'ou no chikara no henrin.
Eien naki, hishou na ryuu no matsuei-tachi ni takai iryoku o hakki-suru.

"Stabbing-sword of gravel containing primordial lightning. A portion being granted from the remembrance of the Dragon King.
This displays the great might of the evanescent and trifling descendants of the dragons."

Following things worth noting:
1) The Japanese description directly mentions that this power belongs to the "trifling descendants of the dragons" (卑小な竜の末裔たち), thus directly stating that the numerous dragons, two-winged and four-winged that dot the land of Elden Ring are the descendant of much older and more powerful dragons akin to the four-headed Placidusax.
2) They are directly described as "evanescent" i.e. not lasting forever, where the literal Japanese wording is 永遠なき (eien-naki, "forever-not"), in relation to Placidusax's temporal suspention and immortality.
3) The sword contains "promordial lightning" (原初の雷), litreally "the first lightning", but "primeval" and "primordial" both refer to the same sense of "of the beginning," "original" and "the first."

Overall the English localisation did a great job at conveying the basic nuances of the Japanese text, but skipping the sense of them being "descendants" unfortunately means a nice detail is lost in regards to how much more fearsome Placidusax is to his offspring.

Placidusax's (sic) Ruin (プラキドサクスの滅び, Purakidosakusu no horobi, "Placidusax' Destruction")

Official English flavour text:

Power gleaned form the remembrance of Dragonlord Placidusax.
Transforms caster into the Dragonlord to spew golden breath from above. This incantation can be cast while jumping.
These are the dying wails of the Dragonlord who once dwelled eternal beyond time.

Japanese original:

竜王プラキドサクスの追憶から得られた力
己が姿を竜王となし、上空から金色のブレスを吐く
ジャンプ中に使うこともできる
それは、時の狭間に永遠に座した竜王の滅びゆく断末魔であった

Ryuu'ou Purakidosakusu no tsuioku kara erareta chikara
Onoga sugata o ryuu'ou to nashi, juukuu kara ougon no buresu o tsuku
Janpu-chuu ni tsukau koto mo dekiru
Sore wa, toki no hazama ni eien ni zashita ryuu'ou no horobi yuku danmatsuma de atta.

"Power attained from the remembrance of Placidusax, the Dragon King.
One’s own form becomes that of the Dragon King, spewing a golden breath from mid-air. This is also useable whilst jumping.
These were the death throes of the Dragon King facing destruction sitting eternally at the interstice of time."

"A few things of interest:
1) His literal title in Japanese is 竜王 (ryuu'ou, "dragon king"), not "lord" as in the English localisation, which has had history with the other Soulsborne games of translating 王 as "lord" rather than "king."
2) The last paragraph needs some glossing:
Interstice of time (時の狭間) - Here literally could also be translated as "temporal interstice" or "temporal space (between)", it is in fact hazama (interstice) that is the term that otherwise is consistenly translated as the "Land Between" in Elden Ring, but is otherwise a regular word in Japanese referring to an interval, interstice or a space between something. Here it means that Placidusax is sitting in a very literal crevice in time, since he is himself a being from another period.
Death throes (断末魔) -  represent in English the expression 断末魔 (dansatsuma, "death throes, hours of death, agony of dath"), whilst literally meaning "judgement-end-demon" it derives from Buddhism.
Facing destruction (滅びゆく) - This is technically the verb horobu (to destroy, to perish, to (fall into) ruin) plus an auxiliary verb that denotes the indicative, i.e. "being destroyed", "facing destruction"). In other words Placidusax appears to be temporally suspended, moribund and as can be seen from his grotesquely scarred body is hardly at the height of his actual power - the player delivering the coup de grace to a dying king of the dragons.
Were (であった) - May seem like a pointless thing to highlight, but in Japanese the existential copula (i.e. "to be") goes at the end of the sentence, in other words since this is placed in the past tense it all the more heightens Placidusax' role as a primeval creature.

Rembrance of the Dragonlord (竜王の追憶, Ryuu'ou no tsuioku, "Remembrance of the Dragon King")

Official English:

Remembrance of Dragonlord Placidusax, hewn into the Erdtree.
The power of its namesake can be unlocked by the Finger Reader. Alternatively, it can be used to gain a great bounty of runes.
The Dragonlord whose seat lies at the heart of the storm beyond time is said to have been Elden Lord in the age before the Erdtree. Once his god was fled, the lord continued to await its return.

Original Japanese:

黄金樹に刻まれた竜王プラキドサクスの追憶
指読みにより、主の力を得ることができる
また、使用により莫大なルーンを得ることもできる
時の狭間、嵐の中心に座す竜王は
黄金樹の前史、エルデの王であったという
だが神は去り、王は帰還を待ち続けていた

Ougonju ni kizamareta ryuu'ou purakidosakusu no tsuioki. Yubiyomi ni yori, nushi no chikara o eru koto ga dekiru. Mata, shiyou ni yori bakudai na ruun o eru koto mo dekiru.
Toki no hazama, arashi no chuushin ni zasu ryuu'ou wa ougonju no zenreki, erude no ou deatta to iu daga kami wa sari, ou wa kikan o machitsudukete ita.

"Remembrance of the Dragon King, Placidusax, carved into the Golden Tree. It is possible to get the power of its owner from the Finger Reader, it is also possible to gain an exceptional amount of ruins.
The Dragon King sitting in the eye of the storm in the temporal interstice, was called an Elden King in the prehistory of the Golden Tree. The god(s) however vanished and thus the King continue to await their homecoming."

Other than slight rephrasings and wordings it is very much the same, apart from the fact that "Erd Tree" is in the original 黄金樹 (ougonju, "Golden Tree" or "Gold Tree"), fitting with the name of the Golden Order (黄金律, ougonritsu, "Law/Order of Gold"). Elden Lord once more uses 王 (ou, "king") as "lord", but the term has historically been gender-neutral, so one could argue that "Elden Ruler" would convey the sense better.

The Comparison between the Gaping Dragon and Placidusax:

Last, but not least, addressing the question in the beginning, in regards to the titles of the Gaping Dragon, in the flavour text of his axe, and that of Placidusax. The weapon is in the English localisation "Dragon King Greataxe" and in the original Japanese version 竜王の大斧, (Ryuu'ou no daifu, "Great Axe of the Dragon King"), which plainly demonstrates that they both have the same epithet.