Highland Tryndamere is a Legacy Skin.
For more read here:
Maybe this condition is also for this skin:
Legacy skins are any skins that were made available temporarily and then retired to the "Legacy Vault." They may be available for normal purchase again at a later date. However,
all Legacy Skins above 520RP are available through Mystery Gifting. These skins only appear in the champion's information tab if you own them.
Hope i can helped.
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