What is the ruling on invoking the dead for help?

The Bible is very clear on the practice of spiritism, which would include trying to communicate with the dead.

The law covenant forbade inquiring of the dead and stated that the practice of spiritism was a capital offense.

(Leviticus 19:31) 31 "'Do not turn to the spirit mediums, and do not consult fortune-tellers so as to become unclean by them. I am Jehovah your God.

(Leviticus 20:6) 6 "'As for the person who turns to the spirit mediums and the fortune-tellers so as to commit spiritual prostitution with them, I will certainly turn against that person and cut him off from his people.

(Deuteronomy 18:9-12) 9 "When you have entered into the land that Jehovah your God is giving you, you must not learn to imitate the detestable practices of those nations. 10 There should not be found in you anyone who makes his son or his daughter pass through the fire, anyone who employs divination, anyone practicing magic, anyone who looks for omens, a sorcerer, 11 anyone binding others with a spell, anyone who consults a spirit medium or a fortune-teller, or anyone who inquires of the dead. 12 For whoever does these things is detestable to Jehovah, and on account of these detestable practices Jehovah your God is driving them away from before you.

And in the time of early Christianity, it was reinforced with the statement that those who practiced spiritism "will not inherit God's Kingdom"

(Galatians 5:19-21) 19 Now the works of the flesh are plainly seen, and they are sexual immorality, uncleanness, brazen conduct, 20 idolatry, spiritism, hostility, strife, jealousy, fits of anger, dissensions, divisions, sects, 21 envy, drunkenness, wild parties, and things like these. I am forewarning you about these things, the same way I already warned you, that those who practice such things will not inherit God's Kingdom.

(Revelation 21:8) 8 But as for the cowards and those without faith and those who are disgusting in their filth and murderers and the sexually immoral and those practicing spiritism and idolaters and all the liars, their portion will be in the lake that burns with fire and sulfur. This means the second death."