Also, don't know how i forgot this... With overlord as commander, unnatural selection and amoeboid changeling should be considered. Quest for renewal and dolmen gate are also very useful in any sliver deck.
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With overlord as your commander, if you keep training grounds in you could have an infinate number of slivers. It is a lengthy combo though. Training grounds + heart sliver + gemhide + queen. Only worth it with overlord as commander because he allows you to search the slivers you need. Wild pair is an absolutely amazing card in a sliver deck.