There are broadly two kinds of derived nominals in Sanskrit ( see [bib]jrb1995[/bib] ):
Primary nominal bases -- derived from verbs by the application of kRudanta affixes. An example is 'ghnataHa' ('those who are killing') derived from the verb 'han:2:P:to kill' (see Stanza 1.35).
Secondary nominal bases -- derived from nominals by the addition of taddhita affixes. An example is 'pitAmahaHa' ('father of father') derived from the nominal 'pitRu' (see Stanza 1.12).
Note that once the above nominal base has been derived, it usually undergoes declension in a manner akin to regular nominals. However, we will see in some cases, that we may need to know the derivation history of a nominal in order to determine the correct declension.