This forum is more for miscellaneous programming. For your case, you'd be best off asking the question in the Triggers & Scripts forum!
In this forum you may ask for help on fixing a trigger or script. But if you need help getting started with a trigger, this is not the right place — use the World Editor Help Zone.
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It'd be good to share your code so far, even if it has errors! But when I was first learning, it was usually easiest to start with GUI and then adapt it to JASS later on (if you choose to do so, GUI is perfectly capable on its own).
At a high level, you would want to issue an order for Onyxia to move to the center of your region. By default, wc3 will play the unit's "Walk" animation during movement. To make her use her "flying" animation (Walk Alternate) instead of the default walk animation, you can add the animation tag "alternate" to the unit (this is the same concept used for characters that morph and should use a new set of animations, e.g. Illidan turning into his Metamorphosis form, or Druids turning into their beast forms).
You can then create a separate trigger to detect when she enters your region, and then you can trigger the "Cinematic Land" animation and remove the "alternate" animation tag.
I've attached a sample map that does what you describe (apart from deleting her). The triggers are listed below:
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Move Onyxia
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Events
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Player - Player 1 (Red) skips a cinematic sequence
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Conditions
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Actions
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Camera - Lock camera target for Player 1 (Red) to Onyxia 0001 <gen>, offset by (0.00, 0.00) using Default rotation
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Animation - Add the alternate animation tag to Onyxia 0001 <gen>
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Set VariableSet TempLoc = (Center of OnyxiaLandRegion <gen>)
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Unit - Order Onyxia 0001 <gen> to Move To TempLoc
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Custom script: call RemoveLocation(udg_TempLoc)
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Onyxia Land
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Events
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Unit - A unit enters OnyxiaLandRegion <gen>
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Conditions
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(Unit-type of (Triggering unit)) Equal to Onyxia
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Actions
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Camera - Reset camera for Player 1 (Red) to standard game-view over 0.00 seconds
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Unit - Order (Triggering unit) to Stop.
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Animation - Remove the alternate animation tag to (Triggering unit)
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Animation - Play (Triggering unit)'s Cinematic Land animation
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Wait 2.60 seconds
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Animation - Reset (Triggering unit)'s animation
From here, if you want to work with JASS, you can select the trigger and go to "Edit > Convert to Custom Text". Or you can continue modifying it in GUI.