The being in question is known by many titled, two of which are Satan which means adversary and Devil which means accuser: he is an adversary and accuser of humanity (for example, see Zechariah 3:1 and Revelation 12:10 for examples). To make a long, folkloric, story short: the Angels are dispatched to Earth and end up falling for human women thus the Genesis 6 affair. Yehuda Shurpin, of the Jewish group Chabad, wrote an interesting article titled, Nephilim: Fallen Angels, Giants or Men? For the reference to Gibborim a footnotes points out that this term “denotes mighty, great” and I know not if by marrow the reference is to the size of the bone itself: in any case, accorind to an 18 inch cubit this would make the thigh bone alone 27 feet. אֲמַר לָהּ תַּרְתֵּיהֶן צְלָוָנָן, מִן גּוֹ דְּאַתְּ הַוְיָא שְׁלָם בְּבֵיתָךְ לָא יְהִי לִיךְ מַחֲזוֹרֵי לְהָכָא, וּמִן גּוֹ דַּהֲוֵי בְּרִיךְ קַיָּם, לָא יִשְׁלֵה וַוי מִפּוּמִיךְ, וַוי דְּלָא שָׁתֵי בְּרִי, וַוי דְּלָא אָכַל בְּרִי, וַוי דְּלָא אָזֵיל בְּרִי לְבֵי כְּנִשְׁתָּא.
רַבָּנָן אָמְרֵי לֹא אֹסִף לִבְנֵי נֹחַ, לֹא אֹסִף לְדוֹרוֹת, אֲנִי אָמַרְתִּי שֶׁתְּהֵא רוּחִי דָּנָה בָּהֶן וְהֵן לֹא בִּקְּשׁוּ, הֲרֵי אֲנִי מְשַׁגְּמָן בְּיִסּוּרִין.

(תהלים צב, טו): עוֹד יְנוּבוּן בְּשֵׂיבָה דְּשֵׁנִים וְרַעֲנַנִּים יִהְיוּ. His father-in-law Rabbi Chia Rabbah said to him: The Holy One of Blessing began to bless you! רַבָּנָן אָמְרֵי מֵהָכָא (דברים כ, יט): כִּי הָאָדָם עֵץ הַשָּׂדֶה, מָה הָאָדָם נוֹתֵן דִּין וְחֶשְׁבּוֹן, אַף עֵצִים נוֹתְנִין דִּין וְחֶשְׁבּוֹן. אָמַר רַבִּי עֲקִיבָא (תהלים י, יג): עַל מֶה נִאֵץ רָשָׁע אֱלֹהִים אָמַר בְּלִבּוֹ לֹא תִדְרשׁ, לֵית דִּין וְלֵית דַּיָּן, אֲבָל אִית דִּין וְאִית דַּיָּן. I was shocked by this, but more because I didn’t think of same-sex marriage as a social issue until the modern era. אָמַר רַבִּי חָנִין אִלּוּ לֹא בָּא אֱלִיהוּא אֶלָּא לְפָרֵשׁ לָנוּ מַעֲשֵׂה יְרִידַת הַגְּשָׁמִים, דַּיּוֹ, דְּאָמַר רַבִּי יוֹחָנָן כָּל אוֹרָה שֶׁנֶּאֱמַר בֶּאֱלִיהוּא אֵינָהּ אֶלָּא בִּירִידַת גְּשָׁמִים. אָמְרָה לוֹ אַבָּא שְׁתֵּי שְׂמָחוֹת שֶׁבָּאוּ לִי אַתָּה מְקַלְּלֵנִי. he wife of Rabbi Shimeon Bar Ami gave birth to a girl. He said to her: Both of them are blessings, if you feel content in your home you wll not long for here, and if your son lives 'vay' will not cease from your lips: 'Vay, my son does not drink enough!' As per Ezekiel 28:14, Satan is a Cherub and not an Angel. A footnote dichotomizes sons of God and Angels plus giants as on the view proposed “benei elokim is understood to refer to princes, noblemen and judges, the term nephilim may very well still be referring to angels and giants” and I would elucidate that the sons of God are Angels and the Nephilim are their giant offspring.