What do i have to do to get the last aetherium shard in skyrim?

You have to go to the Deep Crossing Forest in order to collect the last Aetherium Shard in Skyrim: Lost to the Ages.

Once you are there, travel to the Ruins of Bthalft, which are directly south of Ivarstead and far northwest of Riften. Once there, speak to Katria, who will be waiting for you. Put the shards into the Dwemer Mechanism, then once the crest is assembled, take the last shard out and stand clear. The platform will rise, creating a new entrance way. Use it to descend into the forge.

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