The problem seems to be that the parameters values are not correctly encoded.
As a fast workaround you can simply pass the entire constructed URL.
UNIHTTPJsonResponse* response = [[UNIRest post:^(UNISimpleRequest* request) {
[request setUrl:@"https://api.guildwars2.com/v1/guild_details.json?guild_name=The%20Legacy"];
[request setHeaders:@{@"accept": @"application/json"}];
}] asJson];
Probably the space in @"The Legacy" doesn't translate to "The%20Legacy", will do a test case before adding issue to https://github.com/Mashape/unirest-obj-c
UPDATE
Only while I was adding a TestCase for spaced values (which do work correctly) I spotted that you where using POST while you should have used GET.
UNIHTTPJsonResponse* response = [[UNIRest get:^(UNISimpleRequest* request) {