My favourite card concept

[Vlasaras]

Face-up FIRE monsters you control gain 200 ATK and DEF. During the End Phase of the turn this card was sent from the field to the GY while under your possession by your opponent's card either by battle or card effect: Special Summon this card from your GY. This effect cannot be negated by the effects of face-up cards on the field. You can only control 1 ''Purecinder Phoenix''.

its me favourite, I can just sit here and make phoenix cards all day long..guess ill start small and build bigger phoenix later

[Sleepy]

FIRE monsters you control gain 200 ATK and DEF. If this card you control is destroyed by battle or is sent to the GY by your opponent's effect: Special Summon this card from your GY during the End Phase. This card's activation and effect cannot be negated by the effects of face-up cards. You can only control 1 "Purecinder Phoenix".

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Notes on my grammar changes:

-You don't have to say your FIRE monsters need to be "face-up". If they are face-down, the game cannot confirm what they are to see if they qualify for the boost anyways. Even if they are changed to face-down by an effect, no card in the game can make use of its stats until something flips it back-up. 

-The revival effect is written in a very confusing way. You can state that it happens "during the End Phase" at the resolution part of the text to simplify the condition a bit. Cards like the errata version of Dark Magician of Chaos need to have the extra stuff in the condition section of the effect because that one needs to remain face-up on the field until the End Phase to begin activating, but your Phoenix's trigger requires it to die so such a specification is redundant until later on. 

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Your card is similar to the power of Cerulean Sacred Phoenix of Nephthys in that both are essentially 3k beaters that revive no matter how they were destroyed. Yours a little better as it boosts everything else a little bit and an effect getting rid of it doesn't even need to "destroy" for this to trigger.
Then again, Cerulean is a Ritual as an extra drawback, but Cerulean can destroy stuff with another effect.

The anti-negation clause, I just upped a tiny little bit... 

I don't think the card is broken enough to demand you cannot have more than one on the field at a time. After all, unlike the Fire King or Nephthys cards, this needs to trigger from the field only, and it doesn't do much of anything else. You could have 3 beater floaters of 3400 ATK each, which would be nice and kinda hard earned.... and stuff like Nibiru, Kaijus, Lava Golem would make it stay dead, and anything involving control change like Mind Control, Enemy Controller, the upcoming "3 Tactical Talents", etc. Plus Called by the Grave, D.D. Crow, Transmigration Prophesy, being under Dark Law.... tons of outs.

I think the card is fine.