What is the meaning of 之 and 当 in the phrase "比之当初战火蔓延的速度有过之而无不及"?
之 is acting like a pronoun for the first statement here
[A] = [乱葬岗大围剿刚刚结束，未及第二日，这个消息便插翅一般飞遍了整个修真界]
[比] = [(verb): compare]
[之] = [(pronounce for) [A]: it]
[to] (implied)
[B] = [当初战火蔓延的速度]
[有过之而无不及] = [conclusion]
This sentences are comparing 'the speed it spread' of [A] to [B] and the conclusion is [有过之而无不及] (can only be faster, not slower)

[A] [比] [之] (to) [B] + [conclusion]
[A] [compare] [it] (to) [B] + [conclusion]

A shorter example:
[A] = [違者 (the ones who violate the rule)]
[B] = [斬 (execute)]

違者斬(之) = the ones who violate the rule,  execute (him)
之 here is a acting like a pronounce &quot;him&quot; for 違者

I said 之 is acting like a pronounce because it is also acting like a  preposition 'to' for the verb
&quot;違者斬(之)&quot; can be interpreted as &quot;(to) execute the ones who violate the rule&quot;
You can omit '之'  and say &quot;違者斬&quot;, just like you can omit 'to' and say &quot;execute the ones who violate the rule&quot;