You may want to redirect clients to a backend server's error page if a specific condition is met. The error page is hosted by the same vServer's backend service. You may configure the responder policy like below:

add responder action Resp_Error_Page_Act redirect "\"/path/to/errorpage.html\"" -responseStatusCode 302
add responder policy Resp_Error_Page_Pol "ANY_Expression_You_Want" Resp_Error_Page_Act
bind lb vserver LB_HTTP_vS -policyName Resp_Error_Page_Pol -priority 100 -gotoPriorityExpression END -type REQUEST

However, above configuration can lead to endless 302 redirects on client side, and browser can say: "ERR_TOO_MANY_REDIRECTS".

Exclude the error page URL from NetScaler responder policy expression. 

set responder policy Resp_Error_Page_Pol -rule "ANY_Expression_You_Want && HTTP.REQ.URL.CONTAINS(\"/path/to/errorpage.html\").NOT"

Problem Cause

When the client is redirected to the error page, the subsequent request may still trigger the same responder policy, causing a redirect loop.