Ultimate Beast of Serket [Written]

[Thomas★Zero]

Ultimate Beast of Serket
lv 10 EARTH
Cannot be Normal Summoned/Set. Must be Special Summoned (from your hand) by banishing all monsters in your opponents GY while you control "Temple of the Kings". This cards name becomes "Mystical Beast of Serket" while it is on the field and in your GY. This cards ATK and DEF become equal to the number of monsters banished by this cards effect x 500. Banish any monster destroyed by battle with this card. If this card would be destroyed by battle or by card effect, you can banish 1 Trap Card from your GY instead. 
ATK/? DEF/?

So Ultimate Beast of Serket is based on the form it took after destroying multiple monsters in the Yugioh anime where this card takes the original Serkets effect and instead uses it to banish all monsters in grave to make up its ATK and DEF. In addition to this it also can protect itself by banishing Trap cards.
 

[Rayfield Lumina]

First thing that comes to mind is that you forgot to specify where is this monster Special Summoned from. I find this version of Serket rather fearsome, as being able to banish all monsters in your opponent's GY and become stronger the more it banishes is a bit overpowered in my opinion, specially for the first time you Special Summon it, even if you need an specific Spell in the field for its Summoning. If this card can be Special Summoned from your GY, you could even keep using its effect over and over to banish all monsters in your opponent's GY, and even if it arrives weak at the field, (say, with 500 ATK/DEF for banishing 1 monster) you could immediately use it as material for a Link. If Serket can only be Special Summoned from the hand, things are much different, though.