How to attach custom PHP handler in WordPress?

Question

WordPress

Good day to all and good luck!!!
I have a WordPress site and everything works well.
Task:
I want to connect a post handler. Type:

wp_header();
// мой обработчик
wp_footer();

Conditions:
If the link is like site.com/post/ochen-horoshiy-post-2015/, then my handler should not interfere and WordPress itself issues a post from its database.
And if the type link is site.com/special-posts/custom-post-2015/, then my handler should turn on, which will receive the "custom-post-2015" parameter and issue the necessary post, in the site design and not from the WordPress database (t .e. intercept the CNC) and not generate a 404 error.
How should my handler be included if the link contains "special-posts"
How to implement it: on a plugin, on a filter, on hooks ??? How ?
Updated:
I went about creating a page template. Created, named, the handler is enabled, everything is OK - site.com/special-posts
But as soon as the link contains a tail - "/custom-post-2015/", then 404 is triggered.
If I'm on the right track, tell me how to intercept (or prevent) the 404 handler in such cases?
ReUpdated:
The solution was found: I added a cnc rewrite rule, created a page template and in the template my handler does what he wants.
Thank you all for your replies)))